[media] em28xx: Fix usb diconnect logic

Now that everything is extension, the usb disconnect logic should
be the same.

While here, fix the device name.

Reviewed-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Tested-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>