ath9k: Fix diversity combining for AR9285

When antenna diversity combining is enabled in the EEPROM,
the initial values for the MAIN/ALT config have to be
programmed correctly. This patch adds it for AR9285.

Since the diversity combining macros are common to all chip
families, remove the redundant AR9285 macros and move the
definitions to phy.h.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>