media: rp1: csi2: Fix csi2_pad_set_fmt()

The CSI-2 subdev's set_fmt currently allows setting the source and sink
pad formats quite freely. This is not right, as the CSI-2 block can only
do one of the following when processing the stream: 1) pass through as
is, 2) expand to 16-bits, 3) compress.

The csi2_pad_set_fmt() should take this into account, and only allow
changing the source side mbus code, compared to the sink side format.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>