* always allocate maximum size txhdr entries
* set the right rx header offset/framesize based on firmware
This still isn't what's completely required for fw 598 support; there's
more to come.
Tested:
* Apple BCM94321MC 11abgn NIC, 11a STA mode, firmware version 4xx.
Obtained from: DragonflyBSD (txhdr entry sizing), fw 598 RX header size (linux b43)