[media] em28xx: convert to videobuf2

This patch converts the em28xx driver over to videobuf2.  It is
likely that em28xx_fh can go away entirely, but that will come in
a separate patch.

[mchehab@redhat.com: fix a non-trivial merge conflict with some VBI
 patches; CodingStyle fixes]

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