rtc: m41t80: remove unneeded checks from m41t80_sqw_set_rate

m41t80_sqw_set_rate will be called with the result from
m41t80_sqw_round_rate, so might as well make
m41t80_sqw_set_rate(n) same as
m41t80_sqw_set_rate(m41t80_sqw_round_rate(n))

As Russell King wrote[1],
"clk_round_rate() is supposed to tell you what you end up with if you
ask clk_set_rate() to set the exact same value you passed in - but
clk_round_rate() won't modify the hardware."

[1]
http://lists.infradead.org/pipermail/linux-arm-kernel/2012-January/080175.html

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-m41t80.c

@@ -490,17 +490,12 @@ static int m41t80_sqw_set_rate(struct clk_hw *hw, unsigned long rate,
 		M41T80_REG_WDAY : M41T80_REG_SQW;
 	int reg, ret, val = 0;
 
-	if (rate) {
-		if (!is_power_of_2(rate))
-			return -EINVAL;
-		val = ilog2(rate);
-		if (val == ilog2(M41T80_SQW_MAX_FREQ))
-			val = 1;
-		else if (val < (ilog2(M41T80_SQW_MAX_FREQ) - 1))
-			val = ilog2(M41T80_SQW_MAX_FREQ) - val;
-		else
-			return -EINVAL;
-	}
+	if (rate >= M41T80_SQW_MAX_FREQ)
+		val = 1;
+	else if (rate >= M41T80_SQW_MAX_FREQ / 4)
+		val = 2;
+	else if (rate)
+		val = 15 - ilog2(rate);
 
 	reg = i2c_smbus_read_byte_data(client, reg_sqw);
 	if (reg < 0)