ath9k: Allow platform override without EEPROM override

Add a new platform data flag "use_eeprom" that indicates that the eeprom
found on the card itself should be used instead of the one present in
the platform data.

This allows to override the MAC address of a PCI card while preserving
the eeprom data from the card itself.

The default behavior is preserved.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>