iio: sx9500: fix bug in compensation code

The initial compensation was mistakingly toggling an extra bit in the
control register.  Fix this and make sure it's less likely to happen by
introducing an additional macro.
Reported-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Vlad Dogaru <vlad.dogaru@intel.com>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/proximity/sx9500.c

@@ -80,6 +80,7 @@
 #define SX9500_COMPSTAT_MASK		GENMASK(3, 0)
 
 #define SX9500_NUM_CHANNELS		4
+#define SX9500_CHAN_MASK		GENMASK(SX9500_NUM_CHANNELS - 1, 0)
 
 struct sx9500_data {
 	struct mutex mutex;
@@ -802,8 +803,7 @@ static int sx9500_init_compensation(struct iio_dev *indio_dev)
 	unsigned int val;
 
 	ret = regmap_update_bits(data->regmap, SX9500_REG_PROX_CTRL0,
-				 GENMASK(SX9500_NUM_CHANNELS, 0),
-				 GENMASK(SX9500_NUM_CHANNELS, 0));
+				 SX9500_CHAN_MASK, SX9500_CHAN_MASK);
 	if (ret < 0)
 		return ret;
 
@@ -823,7 +823,7 @@ static int sx9500_init_compensation(struct iio_dev *indio_dev)
 
 out:
 	regmap_update_bits(data->regmap, SX9500_REG_PROX_CTRL0,
-			   GENMASK(SX9500_NUM_CHANNELS, 0), 0);
+			   SX9500_CHAN_MASK, 0);
 	return ret;
 }