drivers: adc: lmp90xxx: fix checksum mismatch return value

During channel reads, zero is returned on CRC mismatches: the returned
error variable is not written to after a previous non-zero check. Return
-EIO to mirror other drivers' checksum validation behaviors.

Signed-off-by: Jared Kangas <kangas.jd@gmail.com>
(cherry picked from commit 8ec3c045)