net80211: create accessors for accessing the ieee80211_key key/mic data

Add some accessors to the key data, key length and MIC data.
Document exactly what these mean.

There's at least a couple of drivers that access the key data field
directly and assume that the TX/RX MIC is available directly after the
data pointer, which bakes in the "key size is 128 bits" in subtle ways.

The goal here is to migrate the drivers and net80211 code to use
these methods rather than accessing wk_key directly and making assumptions
about wk_key and the copied key length (which the ioctl path definitely
does.)

Once that's done, it should be a lot easier to change the key API for
larger keys.

Differential Revision: https://reviews.freebsd.org/D52711
Reviewed by: thj