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

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 <bruno.stephan@usp.br>
Signed-off-by: Bruno Stephan <bruno.stephan@usp.br>
Co-developed-by: Andre de Lima <aschwarz@usp.br>
Signed-off-by: Andre de Lima <aschwarz@usp.br>
Signed-off-by: Arthur Pilone <art.pilone@gmail.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://patch.msgid.link/20250421145534.91146-1-arthurpilone@usp.br
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>