V4L/DVB (8010): em28xx: Properly register extensions for already attached devices

em28xx-video.c
 - Properly handle loading of the module when multiple devices are already
   connected (such as at bootup).  Before we were only calling dvb_init()
   against the last device in the list, so while we were handling subsequent
   adds properly, if there were multiple devices present on driver load,
   everybody except the last device would not get initialized.

Signed-off-by: Devin Heitmueller <devin.heitmueller@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>