iwm: reorganize if_iwmvar.h

[avos]

Summary

• 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;

Test Plan

(untested, compilable)