drivers: iio: imu: Fix NULL pointer dereference in IMU drivers

The early buffer feature enablement for all 3 sensors SMI130, ASM330
and IAM20680 does not has synchronisation between early buffer read
and write, due to that when user application deletes the memory allocated
for early buffer data,the driver still try to write the data which inturn
lead to NULL pointer derefernce.The issue is solved by proper
synchronisation between read and write to early buffer memory by
using mutex lock.

Change-Id: Iea3421da43ad4586bcdde439d0d9dda6522d9474
Signed-off-by: Puneet Yatnal <puneet@codeaurora.org>