Flesh out some more of the DMA init path - based on a really dirty combination
of the linux driver, iwm OpenBSD code and the iwn driver.

Add in the FreeBSD-specific busdma code into if_iwa_trans.c, replacing
the OpenBSD specific bits.  I think the right thing to do here in the
longer term is to put the OS and wifi specific layer bits in separate
source files, keeping the driver as generic as possible.

This does attach/detach but I haven't verified yet whether it's at all
doing the right thing with regards to the DMA mapping setup.  Specifically,
I haven't yet tried firing up the firmware interface and throwing
firmware into the NIC just yet.