[media] em28xx-audio: disconnect before freeing URBs

URBs might be in usage. Disconnect the device before freeing
them.

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>