iwn(4) debugging improvements.

* Add in some new register debugging under IWN_DEBUG_REGISTER
* Make IWN_DEBUG an option now for building.  I'll chase this up
  with a commit to 'options' soon.

Submitted by:	Cedric GROSS <cg@cgross.info>
Add IWN_DEBUG as an option for if_iwn.
Now that conf/options knows about if_iwn.h, add it to if_iwn.c.

This allows for IWN_DEBUG (and maybe more stuff later) to be a build
time configure option.