media: v4l2-fwnode: cleanup functions that parse endpoints

There is already a typedef for the parse endpoint function.
However, instead of using it, it is redefined at the C file
(and on one of the function headers).

Replace them by the function typedef, in order to cleanup
several related coding style warnings.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>