iio: st_sensors: Stop abusing mlock to ensure internal coherency

An odr_lock has been introduced to protect local accesses to the odr
internal cache and ensure the cached value always reflected the actual
value. Using the mlock() for this purpose is no longer needed, so let's
drop these extra mutex_lock/unlock() calls.

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Cc: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20220207143840.707510-8-miquel.raynal@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>