[net80211] turn the default TX key configuration (for WEP) into a vap callback.

The ath10k firmware supports hardware WEP offload, and in native wifi mode
(or 802.3 ethernet mode, for that matter) the WEP key isn't actually included
in the TX payload from net80211. Instead, a separate firmware command is issued
that sets the default TX key to be the specified key.

However, net80211 doesn't at all inform the driver layer that this is
occuring - it just "expects" to be inserting WEP header information
when doing WEP TX, even with hardware encryption.

So, to better support the newer world order, turn the default TX key assignment
into a VAP method that can be overridden by the driver and ensure its wrapped
in a crypto begin/end set. That way it should be correctly atomic from the
point of view of keychanges (as long as the driver does the right thing.)

It'd be nice if we passed through to the key_set call a flag that says
"also make this the default key" - that's captured here by calling the
deftxkey method after the key_set method. Maybe I can do that later.

Note: this is a net80211 ABI change, and will require a kernel+modules
recompile. Happy Holidays, etc.

Tested:

• ath10k driver port
• rtwn_usb, WEP station