V4L/DVB: v4l2-common: simplify prio utility functions

v4l2_prio_init/open/close returned an int when in fact they would
always return 0. Make these void functions.

v4l2_prio_close and v4l2_prio_check pass an enum v4l2_priority as a
pointer for no good reason. Replace with a normal enum v4l2_priority
argument.

These changes will simplify the work of moving priority handling into
the v4l core.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>