V4L/DVB (9743): em28xx: fix oops audio

Replaced usb_kill_usb for usb_unlink_usb
(wait until urb to fully stop require USB core to put the calling process to sleep).

Oops:
http://www.kerneloops.org/raw.php?rawid=71799&msgid=

Signed-off-by: Douglas Schilling Landgraf <dougsland@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>