rt2800: prepare for rt2800*_probe_hw_mode() unification

Enclose interface specific code in rt2800[pci,usb]_probe_hw_mode()
with rt2x00_intf_is_[pci,usb]() checks.

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