Prepare for the PAN (personal area network) support for iwn(4).

* Break out the single, static RX context into a pointer, and ..
* .. extend it to two RX contexts - a default and a PAN context.

Whilst here, add a few extra fields in preparation for further iwn(4)
work.

Tested:

* Intel 4965, STA mode - same level of stability
* Intel 5100, STA mode - no change

Submitted by:	Cedric Gross <cg@gross.info>