iio: adc: ad7091r-base: Remove duplicate code on volatile reg check

Both ad7091r_writeable_reg() and ad7091r_volatile_reg() perform the
same test, checking whether a given 'reg' code is
AD7091R_REG_RESULT or AD7091R_REG_ALERT. As the volatile ad7091r
registers happen to be the only read-only ones, the volatile_reg()
function now returns the negated output of writeable_reg().

Co-developed-by: Bruno Stephan <[e-mail]>
Signed-off-by: Bruno Stephan <[e-mail]>
Co-developed-by: Andre de Lima <[e-mail]>
Signed-off-by: Andre de Lima <[e-mail]>
Signed-off-by: Arthur Pilone <[e-mail]>
Reviewed-by: Marcelo Schmitt <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/ad7091r-base.c

···
 
 bool ad7091r_volatile_reg(struct device *dev, unsigned int reg)
 {
-	switch (reg) {
-	case AD7091R_REG_RESULT:
-	case AD7091R_REG_ALERT:
-		return true;
-	default:
-		return false;
-	}
+	/* The volatile ad7091r registers are also the only RO ones. */
+	return !ad7091r_writeable_reg(dev, reg);
 }
 EXPORT_SYMBOL_NS_GPL(ad7091r_volatile_reg, "IIO_AD7091R");