iwlegacy: make array interval static, makes object smaller

Don't populate the array interval on the stack but instead make it
static. Makes the object code smaller by 121 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
 167797	  29676	    448	 197921	  30521	wireless/intel/iwlegacy/common.o

After:
   text	   data	    bss	    dec	    hex	filename
 167580	  29772	    448	 197800	  304a8	wireless/intel/iwlegacy/common.o

(gcc version 9.2.1, amd64)

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