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: I731a015c952d131187a47a8d346fb6478fddeeb1
Signed-off-by: Samyukta Mogily <smogily@codeaurora.org>