iwlegacy: move some i/o helpers out of inline

This save us about 20k of text size, and should have no impact on
performance as hot paths do not use much I/O.

Before:
   text	   data	    bss	    dec	    hex	filename
 108512	   1784	    168	 110464	  1af80	drivers/net/wireless/iwlegacy/iwl3945.ko
 165730	   2164	    156	 168050	  29072	drivers/net/wireless/iwlegacy/iwl4965.ko
  91942	    328	     48	  92318	  1689e	drivers/net/wireless/iwlegacy/iwlegacy.ko

After:
   text	   data	    bss	    dec	    hex	filename
  95556	   1784	    168	  97508	  17ce4	drivers/net/wireless/iwlegacy/iwl3945.ko
 154853	   2164	    156	 157173	  265f5	drivers/net/wireless/iwlegacy/iwl4965.ko
  91634	    328	     48	  92010	  1676a	drivers/net/wireless/iwlegacy/iwlegacy.ko

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>