ath6kl: fix incorrect use of IEEE80211_NUM_BANDS

ath6kl was incorrectly assuming that IEEE80211_NUM_BANDS will always be 2
and used that also in the firmware WMI interface definitions. But after
the support for 60 GHz was added to cfg80211 IEEE80211_NUM_BANDS changed to 3
and this can cause all sort of problems, possibly even memory corruption.
I only found this during code review and didn't notice any bugs, but I'm
sure there are a few lurking somewhere.

To fix this rename unused A_NUM_BANDS to ATH6KL_NUM_BANDS, which is
always defined to be 2, and use that in WMI.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>