iio:potentiostat:lmp91000: invert if statement

Invert if statement arms in line 214, in order to make the code cleaner,
solve these checkpatch.pl CHECKs:

- lmp9100.c:214: CHECK: braces {} should be used on all arms of this statement
- lmp9100.c:216: CHECK: Unbalanced braces around else statement

Signed-off-by: Lucas Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: Anderson Reis <andersonreisrosa@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>