msm: camera: Fix for post power-down actuator IOCTLs

When there is an error condition, there is a possibility
for the call sequence to be out of order. Hence the actuator
IOCTLs could be invoked after power-down. This NULL assignment
ensured no IOCTLs are processed by the actuator.

Change-Id: I4cf228212220633e4f9d978e3871e735f8f360ef
Signed-off-by: Rajakumar Govindaram <rajakuma@codeaurora.org>