staging: iio: ad7152: Fix deadlock in ad7152_write_raw_samp_freq()
ad7152_write_raw_samp_freq() is called by ad7152_write_raw() with
chip->state_lock held. So, there is unavoidable deadlock when
ad7152_write_raw_samp_freq() locks the mutex itself.
The patch removes unneeded locking.
Found by Linux Driver Verification project (linuxtesting.org).