[bwn] add initial 5xx firmware API support

• Add the new TX/RX frame formats;
• Use the right TX/RX format based on the frame info;
• Disable the 5xx firmware check, since now it should somewhat work (but note, we don't yet use it unless you manually add ucode11/initvals11 from the 5.x driver to bwn-kmod-firmware;

• Misc: update some comments/debugging now I know what's actually going on.

Tested:

• BCM4321MC, STA mode, both 4xx and 666 firmware, DMA mode

TODO:

• The newer firmware ends up logging "warn: firmware state (0)"; not sure yet what's going on there. But, yes, it still works. I'm committing this via a BCM4321MC, 11a station, firmware rev 666.

Obtained from: Linux b43 (TX/RX descriptor format for 5xx)