staging: comedi: das08: replace DAS08AO_GAIN_CONTROL/STATUS

The `DAS08AO_GAIN_CONTROL` and `DAS08AO_GAIN_STATUS` macros hold the
offset to the "programmable gain" register on "PGL", "PGM", "PGH",
"AOL", "AOM" and "AOH" boards.  Writing a code to this register sets the
gain for the current analog input channel (selected in the main control
register).  The written value can be read back in bits 3..0 of the
register.  Other bits of the register are read-only and not used by the
driver.  Rename `DAS08AO_GAIN_CONTROL` to `DAS08_GAIN_REG` and add a
comment.  Remove `DAS08AO_GAIN_STATUS` as the driver does not use it and
the read-only parts of the register are documented in the comment.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/das08.c

@@ -78,6 +78,13 @@
 #define DAS08_CONTROL_DO_MASK	0xf0	/* digital outputs mask (not "JR") */
 /* digital outputs (not "JR" boards) */
 #define DAS08_CONTROL_DO(x)	(((x) << 4) & DAS08_CONTROL_DO_MASK)
+/*
+ * (R/W) programmable AI gain ("PGx" and "AOx" boards):
+ * + bits 3..0 (R/W) show/set the gain for the current AI mux channel
+ * + bits 6..4 (R) show the current AI mux channel
+ * + bit 7 (R) not unused
+ */
+#define DAS08_GAIN_REG		0x03
 
 /*
     cio-das08jr.pdf
@@ -127,9 +134,6 @@
   cdef	8255
 */
 
-#define DAS08AO_GAIN_CONTROL	3
-#define DAS08AO_GAIN_STATUS	3
-
 #define DAS08AO_AO_LSB(x)	((x) ? 0xa : 8)
 #define DAS08AO_AO_MSB(x)	((x) ? 0xb : 9)
 #define DAS08AO_AO_UPDATE	8
@@ -260,7 +264,7 @@ static int das08_ai_rinsn(struct comedi_device *dev, struct comedi_subdevice *s,
 		/* set gain/range */
 		range = CR_RANGE(insn->chanspec);
 		outb(devpriv->pg_gainlist[range],
-		     dev->iobase + DAS08AO_GAIN_CONTROL);
+		     dev->iobase + DAS08_GAIN_REG);
 	}
 
 	for (n = 0; n < insn->n; n++) {