Since USB receives STATUS_RSSI updates through the interrupt endpoint,
there is no need to poll with USB, so get rid of the polling.

Note this also changes the order in which the probing of USB devices is done,
to avoid si470x_set_chan getting called before the interrupt endpoint is being
monitored.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>