msm: camera_v3: sensor: Subtract offset before validating cmd desc

Subtract command descriptor offset from buffer length obtained
from kernel before validating length of command buffer.

Change-Id: I38b244873c20894ee26b687c4bb19ef103c79363
Signed-off-by: Mukund Madhusudan Atre <matre@codeaurora.org>
Signed-off-by: Raja Mallik <rmallik@codeaurora.org>