rt2x00: Add RFKILL support to rt2500usb and rt73usb

Some very rare Ralink USB hardware exists which features
the RFKILL switch on the USB stick.
This patch adds the EEPROM check function to see if RFKILL
is supported and the polling function to rt2500usb and
rt73usb in order to support RFKILL for that hardware.

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