media: s5p-mfc: Use standard frame skip mode control

Use the standard menu control for frame skip mode in the MFC
driver. The legacy private menu control is kept for backward
compatibility.

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>