Use the new net80211 method to fetch the node TX power, rather than
directly referencing ni->ni_txpower.

This provides the hardware with a slightly more accurate idea of
the maximum TX power to be using.

This is part of a series to get per-packet TPC to work (better).

Tested:

* AR5416, hostap mode
r249408 and r249436 cause a NULL pointer dereference on the CUBIEBOARD
since it doesn't set the kernel envrionment at all. Work around this
by making sure kern_envp is non-NULL before dereferencing it.