iio: adc: at91: signedness bug in at91_adc_get_trigger_value_by_name()

at91_adc_get_trigger_value_by_name() was returning -ENOMEM truncated to
a positive u8 and that doesn't work.  I've changed it to int and
refactored it to preserve the error code.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Tested-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: Stable@vger.kernel.org