net: phy: mscc: make arrays static, makes object smaller

Don't populate const arrays on the stack but instead make them
static. Makes the object code smaller by 1058 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
  29879	   6144	      0	  36023	   8cb7	drivers/net/phy/mscc.o

After:
   text	   data	    bss	    dec	    hex	filename
  28437	   6528	      0	  34965	   8895	drivers/net/phy/mscc.o

(gcc version 9.2.1, amd64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>