[bwn] add in bwn n-phy linking.

• The default kernel and options won't build the GPL PHY bits;
• bwn(4) defaults to building as a module anyway!;
• If BWN_GPL_PHY is specified in the config file, and you uncomment the GPL PHY bits in the module Makefile, you'll get a working N-PHY.

This is specifically designed to be obtuse for now, as I don't want
to flip it on by default. It's easy enough for people to flip on
and build, and it's a module so the default GENERIC kernel won't be
GPL tainted.

I'll have to add an actual HAL layer that allows the GPL PHY to be loaded
before if_bwn so it can be "magic", but that'll come later.

Tested:

• BCM4321 11abg NIC, STA mode