ath9k_hw: add silicon revision macros for AR9300

AR9300 will be the first device supported of the AR9003
family. AR9300 1.0 hardware exists but it is not going to
be sold anywhere so we completely skip its support.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>