iwlegacy: make const array static to shink object code size

Don't populate const array ac_to_fifo on the stack in an inlined
function, instead make it static.  Makes the object code smaller
by over 800 bytes:

   text	   data	    bss	    dec	    hex	filename
 159029	  33154	   1216	 193399	  2f377	4965-mac.o

   text	   data	    bss	    dec	    hex	filename
 158122	  33250	   1216	 192588	  2f04c	4965-mac.o

(gcc version 7.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>