drivers: iio: imu: Fixed minor bugs of IAM20680 sensor

Fixed NULL pointer dereference of sensor IAM20680 in initalization
of early buffer feature and avoid writing to any syfs entry from
userspace while buffering IAM sensor data because which leads
to buffer data loss.

Change-Id: I2014145855ed7e30e2e829612b0fd3d64f0de7df
Signed-off-by: Puneet Yatnal <puneet@codeaurora.org>