mwifiex: make const array tos_to_ac static, reduces object code size

Don't populate the read-only const array tos_to_ac on the stack,
instead make it static. Makes the object code smaller by 250 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  26104	   2720	    128	  28952	   7118	wmm.o

After:
   text	   data	    bss	    dec	    hex	filename
  25758	   2816	    128	  28702	   701e	wmm.o

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