rt2800: prepare for unification of EEPROM support code

* Factor out common code from rt2800[pci,usb]_validate_eeprom()
  to rt2800_validate_eeprom().

* Fix interface specific comment in rt2800[pci,usb]_validate_eeprom().

* Enclose interface specific code in rt2800[pci,usb]_init_eeprom()
  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>