[media] s5p_mfc: unify variable naming style

Variable frame_size represents the size of plane luminance
here, not just frame size, its naming style should be unified
as frame_size_ch and frame_size_mv.

Signed-off-by: Zhaowei Yuan <zhaowei.yuan@samsung.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>