[media] em28xx: add em28xx_ prefix to functions

Makes it more straight-forward to follow stack traces if the functions
don't have generic names. Using this as a crutch while trying to better
understand the lockdep warnings I get when loading the em28xx driver.

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