ath9k_hw: fix RF bank initialization

ar900*_init_mode_regs needs to be called before RF banks are allocated,
otherwise the storage size of RF banks isn't known. This patch fixes
a memory overrun that can show up as a crash on unloading the module.

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