media: ti-vpe: cal: Don't store external rate in cal_camerarx

The external pixel rate is retrieved when starting the camerarx and only
used then. There's no need to store it in the cal_camerarx structure, it
can be returned by cal_camerarx_get_external_info() and explicitly
passed to cal_camerarx_config().

While at it, rename cal_camerarx_get_external_info() to
cal_camerarx_get_external_rate() to better reflect the function's
purpose.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>