rt2x00: add support for different chipset interfaces

Enhance rt2x00 infrastructure by adding explicit information
about used chipset interface to struct rt2x00_chip.

The new field will be used by rt2800 drivers for rt2800 library.

Also add commonly used rt2x00_intf_is_pci() and rt2x00_intf_is_usb()
helpers to make code easier to read (noticed by Ivo van Doorn).

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>