wl12xx: cmd and acx interface rework

Rework cmd and acx interfaces, it was just too confusing earlier. Now
all commands need to contain all the needed headers, either just cmd
headers or both cmd and acx headers. This accomplish to remove the
extra copy done for each command. The interfaces are now properly
documented as well.

Also try to make all commands safe for DMA transfers. I might have missed
some, but most of them should be fixed now.

And this is not all! As a free bonus you will also get some cosmetic
cleanups and code reorganisation. Order today!

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>