brcmsmac: make const array ucode_ofdm_rates static, reduces object code size

Don't populate const array ucode_ofdm_rates on the stack, instead make it
static. Makes the object code smaller by 100 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  39482	    564	      0	  40046	   9c6e	phy_cmn.o

After
   text	   data	    bss	    dec	    hex	filename
  39326	    620	      0	  39946	   9c0a	phy_cmn.o

(gcc 6.3.0, x86-64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>