wlan - Sync dev/netif/bwn/bwn from FreeBSD
* Sync dev/netif/bwn/bwn from FreeBSD, fbsd git
dd885b9a0a0e, May 11 2016.
* Straight report from FreeBSD using new infrastructure.
* Add bwn back to build
* Using contigmalloc for allocating dr_txhdr_cache, because kmalloc on
DragonFly wouldn't always give us physically contiguous memory.
* Keeping the old code for (MSI) interrupt allocation, which puts things
into the softc struct, and not into the bwn_mac struct like the new code.
* XXX Not tested yet, also some local changes might be missing still.
* XXX Didn't sync dev/netif/bwn/siba_bwn from FreeBSD yet, but bwn might
already work for the previously supported chipsets like this.