net/mlx4_en: ethtool: make array modes static const, makes object smaller

Don't populate the array modes on the stack but instead make it
static const. Makes the object code smaller by 303 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
  51240	   5008	   1312	  57560	   e0d8 mellanox/mlx4/en_ethtool.o

After:
   text	   data	    bss	    dec	    hex	filename
  50937	   5008	   1312	  57257	   dfa9	mellanox/mlx4/en_ethtool.o

(gcc version 9.2.1, amd64)

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