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[bwn] add initial bwn(4) N-PHY code, ported from Linux b43.

This is a GPLv2 PHY-N implementation based on the Linux b43 driver,
ported over to work in bwn(4).

I've tested this on the BCM4321 11abgn device, in 11bg and 11a modes.
The b43 PHY code only supports 11abg, no 11n, and 20MHz only wide
channels.

Yes, this is a GPLv2 driver, so it won't be included in the
default builds.

Tested:

* BCM4321 11abgn device (Apple!), 11bg and 11a STA mode.

Obtained from: Linux b43