media: uvcvideo: Remove a redundant check

Event subscribers cannot have a NULL file handle. They are only added
at a single location in the code, and the .fh pointer is used without
checking there.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@intel.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>