[bwn] break out the LP PHY code into a separate file.

This (and eventually migrating the other PHY code out) is in preparation
for adding the 11n PHY. No, the 11ac PHY (for the BCM4260 softmac part) isn't
yet open source, so we can't grow that. Yet.

This trims ~3,700 lines of code from if_bwn.c, bringing it down to a slightly
less crazy sounding 10,446 lines of code.