msm: vidc: Fix a bug in handling corrupt/unknown packets

The wrong variable (which was the output parameter) was getting
decremented when we encountered an unknown packet.  We should be
decrementing the local packet_count variable.

Change-Id: I32c09e7b7bbfaef5ae92a74d70e4422f7d3b1649
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>