[media] em28xx: refactor the code in em28xx_usb_disconnect()

The main purpose of this patch is to move the call of em28xx_release_resources()
after the call of em28xx_close_extension().
This is necessary, because some resources might be needed/used by the extensions
fini() functions when they get closed.
Also mark the device as disconnected earlier in this function and unify the
em28xx_uninit_usb_xfer() calls for analog and digital mode.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>