rt2x00: Merge rt2800{pci/usb} radio enabling/disabling code to rt2800lib

The functions rt2800pci_enable_radio and rt2800usb_disable_radio are
almost equal and can be merged into rt2800lib. This reduces the number
of functions which must be exported from rt2800lib to the drivers at
the same time.

Also rt2800pci_disable_radio and rt2800usb_disable_radio are almost
equal and can be merged into rt2800lib in a similar fashion.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>