media: rcar-csi2: Optimize rcsi2_calc_mbps()

media: rcar-csi2: Optimize rcsi2_calc_mbps()

With modern drivers supporting link-freq, we don't need to do any
calculations based on the bpp and number of lanes when figuring out the
link frequency. However, the code currently always runs code to get the
bpp and number of lanes.

Optimize the rcsi2_calc_mbps() so that we only do that when needed, i.e.
when querying the link-freq is not supported by the upstream subdevice.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Tested-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>