msm: mdss: hdmi-cec: bounds check cec frame size

Ensure that the frame size for the HDMI-CEC message
frame size doesn't exceed the maximum size when
reading message from userspace through sysfs interface.
This avoids potential overflow in cec send msg routine.

Change-Id: I891f23ed70ed5a6c0e3af1f47869effa079ffac8
CRs-Fixed: 520140
Signed-off-by: Manoj Rao <manojraj@codeaurora.org>