rt2x00: Clean up use of rt2x00_intf_is_pci.

RT chipsets are unique across both PCI and USB busses, and don't overlap.
Therefore there is no need to test for bus type when only checking for
chipset type. Remove the redundant checks.

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>