rt2x00: rt2800 - Make probe_hw function common between PCI and USB.

Refactor the probe_hw code so that more code can be shared between
rt2800pci and rt2800usb.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>