V4L/DVB (12744): em28xx: restructure fh/dev locking to handle both video and vbi

The current locking infrastructure didn't support having multiple fds accessing
the device (such as video and vbi).  Rework the locking infrastructure,
borrowing the design from cx88.

This work was sponsored by EyeMagnet Limited.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>