Don't populate the const array rate_table on the stack but instead it
static. Makes the object code smaller by 46 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
  29812	   3824	    192	  33828	   8424	drivers/net/dsa/bcm_sf2.o

After:
   text	   data	    bss	    dec	    hex	filename
  29670	   3920	    192	  33782	   83f6	drivers/net/dsa/bcm_sf2.o

(gcc version 10.2.0)
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20201020165029.56383-1-colin.king@canonical.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>