ath9k: add MAC revision detection for AR9330

The AR9330 1.0 and 1.1 are using the same revision,
thus it is not possible to distinguish the two chips.
The platform setup code can distinguish the chips based
on the SoC revision.

Add a callback function to ath9k_platform_data in order
to allow getting the revision number from the platform code.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>