staging: iio: frequency: ad9834: Remove unnecessary parentheses

Remove unneeded parentheses around the arguments of ||. This reduces
clutter and code behave in the same way. Change suggested by
checkpatch.pl.

CHECK: Unnecessary parentheses around 'st->devid == ID_AD9833'
CHECK: Unnecessary parentheses around 'st->devid == ID_AD9837'

Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>