V4L/DVB (12743): em28xx: fix mmap_mapper with vbi

When adding support for both video and VBI, I missed the mmap ioctl.  Add
the missing call.

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