drivers: iio: imu: Add mutex lock to ensure proper iio buffer access

Add mutex lock in writing data to iio buffer to avoid iio buffer
access by other api which resulting in null pointer dereference.

Change-Id: I336a9358ff1bd2352d41e633a5a0629026596fa6
Signed-off-by: Puneet Yatnal <puneet@codeaurora.org>