[media] tc358743: Add enum_mbus_code

There was no way to query the supported mbus formats from this
driver. enum_mbus_code is the function to expose that, so
implement it.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>