iwlwifi: move agn specific station code there

By duplicating a little bit of code between 3945
and agn, we can move a lot of code into an agn
specific station management file and thus reduce
the amount of code in core that is dead to 3945.

before:
   text	   data	    bss	    dec	    hex	filename
 212886	   3872	     96	 216854	  34f16	iwlcore.ko
 620542	  10448	    304	 631294	  9a1fe	iwlagn.ko
 314013	   3264	    196	 317473	  4d821	iwl3945.ko

after:
   text	   data	    bss	    dec	    hex	filename
 202857	   3872	     92	 206821	  327e5	iwlcore.ko
 629102	  10448	    308	 639858	  9c372	iwlagn.ko
 314240	   3264	    196	 317700	  4d904	iwl3945.ko

delta:
 -10029   iwlcore.ko
   8560   iwlagn.ko
    227   iwl3945.ko

so it's a net win even if you have both loaded,
likely because a lot of EXPORT_SYMBOLs go away.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>