media/bcm2835-unicam: Parse pad numbers correctly

The driver was making big assumptions about the source device
using pad 0 and 1, which doesn't follow for more complex
devices where Unicam's source device may be a sink device for
something else.

Read the pad numbers through media controller, and reference
them appropriately.

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