media: bcm2835-unicam: Fixup for 5.18 and new get_mbus_config struct

The number of active CSI2 data lanes has moved within the struct
v4l2_mbus_config used by the get_mbus_config API call.
Update the driver to match the changes in mainline.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>