net: stmmac: make const array route_possibilities static

Don't populate array route_possibilities on the stack but make it
static const.  Makes the object code a little smaller by 85 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
   9901	   2448	      0	  12349	   303d	dwmac4_core.o

After:
   text	   data	    bss	    dec	    hex	filename
   9760	   2504	      0	  12264	   2fe8	dwmac4_core.o

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>