drm/i2c: tda9950: Remove VLA usage

In the quest to remove all stack VLA usage from the kernel[1], this
sets the buffer to maximum size and adds a sanity check.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com



Cc: David Airlie <airlied@linux.ie>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Russell King <rmk+kernel@armlinux.org.uk>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Kees Cook <keescook@chromium.org>