msm: camera: Add error check for cmd in IOCTL.

Return EINVAL if cmd is not v4l2 PRIVATE_IOCTL_CMD.

CRs-Fixed: 2016416
Change-Id: I6758bc9d6b159b2e2e4eba5a9d9a3882de320041
Signed-off-by: Terence Ho <terenceh@codeaurora.org>