powerpc/ps3: Make ps3av_set_video_mode mode ID signed

Change the ps3av_auto_videomode() mode id argument type from unsigned to
signed so a negative id can be detected and reported as an -EINVAL failure.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>