media: i2c: adv748x: Implement get_mbus_config

Upstream https://patchwork.linuxtv.org/patch/64676/



Implement the newly introduced get_mbus_config operation to report the
number of currently used data lanes on the MIPI CSI-2 interface.

Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>