viafb: fix i2c_transfer error handling

i2c_transfer returns negative errno on error and number of messages
processed on success. Just returning this value would give a poor
interface as it is not obvious that you must compare with 2 after reading
1 or n bytes and with 1 after writing 1 byte to determine if it was
successful. To avoid this error prone interface convert the error code
of a successful read/write to zero and all other non-negative values to
an negative error code.
This fixes a regression introduced by
	via: Rationalize vt1636 detection
which resulted in no longer detecting a VT1636 chip and therefore has
broken the output in configurations which contain this chip.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Cc: Joseph Chan <JosephChan@via.com.tw>
Cc: stable@kernel.org

drivers/video/via/via_i2c.c

@@ -114,6 +114,7 @@ static void via_i2c_setsda(void *data, int state)
 
 int viafb_i2c_readbyte(u8 adap, u8 slave_addr, u8 index, u8 *pdata)
 {
+	int ret;
 	u8 mm1[] = {0x00};
 	struct i2c_msg msgs[2];
 
@@ -126,11 +127,18 @@ int viafb_i2c_readbyte(u8 adap, u8 slave_addr, u8 index, u8 *pdata)
 	mm1[0] = index;
 	msgs[0].len = 1; msgs[1].len = 1;
 	msgs[0].buf = mm1; msgs[1].buf = pdata;
-	return i2c_transfer(&via_i2c_par[adap].adapter, msgs, 2);
+	ret = i2c_transfer(&via_i2c_par[adap].adapter, msgs, 2);
+	if (ret == 2)
+		ret = 0;
+	else if (ret >= 0)
+		ret = -EIO;
+
+	return ret;
 }
 
 int viafb_i2c_writebyte(u8 adap, u8 slave_addr, u8 index, u8 data)
 {
+	int ret;
 	u8 msg[2] = { index, data };
 	struct i2c_msg msgs;
 
@@ -140,11 +148,18 @@ int viafb_i2c_writebyte(u8 adap, u8 slave_addr, u8 index, u8 data)
 	msgs.addr = slave_addr / 2;
 	msgs.len = 2;
 	msgs.buf = msg;
-	return i2c_transfer(&via_i2c_par[adap].adapter, &msgs, 1);
+	ret = i2c_transfer(&via_i2c_par[adap].adapter, &msgs, 1);
+	if (ret == 1)
+		ret = 0;
+	else if (ret >= 0)
+		ret = -EIO;
+
+	return ret;
 }
 
 int viafb_i2c_readbytes(u8 adap, u8 slave_addr, u8 index, u8 *buff, int buff_len)
 {
+	int ret;
 	u8 mm1[] = {0x00};
 	struct i2c_msg msgs[2];
 
@@ -156,7 +171,13 @@ int viafb_i2c_readbytes(u8 adap, u8 slave_addr, u8 index, u8 *buff, int buff_len
 	mm1[0] = index;
 	msgs[0].len = 1; msgs[1].len = buff_len;
 	msgs[0].buf = mm1; msgs[1].buf = buff;
-	return i2c_transfer(&via_i2c_par[adap].adapter, msgs, 2);
+	ret = i2c_transfer(&via_i2c_par[adap].adapter, msgs, 2);
+	if (ret == 2)
+		ret = 0;
+	else if (ret >= 0)
+		ret = -EIO;
+
+	return ret;
 }
 
 /*