staging: iio: adc: Remove unnecessary test from if conditions

Remove unnecessary test condition on ret variable which has been
previously tested and returns its value if the value is non zero.

This fixes the following smatch warnings:
drivers/staging/iio/adc/ad7816.c:299 ad7816_set_oti() warn: we tested
'ret' before and it was 'false'
drivers/staging/iio/adc/ad7816.c:306 ad7816_set_oti() warn: we tested
'ret' before and it was 'false'

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>