msm: sensor: Fix for variable being de-referenced without proper check

Pointer from userspace is de-referenced before the command is checked.
This might cause a crash if the command being sent is not a valid command.
Hence changing the de-reference such that the pointer is accessed after
checking if a valid command is sent from the userspace.

Change-Id: Iba2e2da5b5adfa0323f8b7d56445af14acd58f84
CR fixed: 2053970
Signed-off-by: Rahul Sharma <sharah@codeaurora.org>