[ath] Begin using the replacement EDCA functions.

As part of ath10k and other chipset support, the EDCA stuff has to be moved
to potentially be per-VAP. For hardware that doesn't support it (ie,
everything that we currently support) it can just fetch the "current"
global EDCA parameters for the NIC.

This is one of those parameters that is linked to the currently active
channel context / VAP in Linux mac80211 parlance.

Tested:

• ath(4), STA and AP modes