[media] mtk-vcodec: fix sparse warning

This patch fix sparse warning when running with parameters:
C=2 CF="-D__CHECK_ENDIAN__"

Signed-off-by: Tiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: PoChun Lin <pochun.lin@mediatek.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>