staging: iio: cdc: ad7152: Replace mlock with a local mutex lock

mlock is intended to protect only switches between modes.
Given this driver doesn't support more than one mode (sysfs polled reads
only), replace mlock with a local mutex lock.

Signed-off-by: sayli karnik <karniksayli1995@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>