iwm: reorganize if_iwmvar.h

• Change order of data in if_iwmvar.h

(like it is in other drivers: defines, data structures,
vap/node structures, softc struct and locks); use indentation.

• Fix IWM_LOCK(_sc) / IWM_UNLOCK(_sc) macro.
• Add IWM_LOCK_INIT / DESTROY(sc) + fix mtx_init() usage.
• Wrap iwm_node casts into IWM_NODE() macro.
• Drop some fields:
  • wt_hwqueue from Tx radiotap header;
  • macaddr[6] from iwm_vap;

Approved by: adrian
Differential Revision: https://reviews.freebsd.org/D4753