[media] airspy: move out of staging into drivers/media/usb

Move it out of staging into media like all the other SDR drivers
too. There is no good reasons to keep these SDR drivers in staging.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>