[media] em28xx: replug locking cleanup

Simplifies the locking by moving the em28xx_init_extension() call until
em28xx_usb_probe() has finished with the dev->lock mutex. It therefore
makes the second and subsequent "plugging" events logically identical to
the first "plugging" event when the em28xx-dvb and em28xx-alsa modules
must be loaded (i.e. registered).

Basically, em28xx_usb_probe() requests that em28xx-dvb be loaded and
also triggers udev to initialise the V4L2 devices. These two events are
serialised by the dev->lock mutex but the order that they happen in is
undefined. But this has always been the case anyway.

Signed-off-by: Chris Rankin <rankincj@yahoo.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>