media: v4l2-fwnode: use a typedef for a function callback

That allows having a kernel-doc markup for the function
prototype. It also prevents the need of describing the
return values twice.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>