media: rcar-csi2: Use v4l2_get_link_freq()

Instead of directly using V4L2_CID_PIXEL_RATE and calculating the mbps
from that, use v4l2_get_link_freq(), which also supports
V4L2_CID_LINK_FREQ.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>