drivers: iio: imu: Fixed compilaton issue when early buffer not enabled

Fixed the compiltion issue of asm330 where the struct st_asm330lhh_sensor
has no member named 'sensor_buff' when early buffer is not
enabled for asm330.

Change-Id: Ic3858b710b7ba53498d38c632a5b851a8fa40e68
Signed-off-by: Puneet Yatnal <puneet@codeaurora.org>