net: ethernet: ti: cpsw: make array stpa static const, makes object smaller

Don't populate the array stpa on the stack but instead it
static const. Makes the object code smaller by 81 bytes:

Before:
   text    data   bss    dec    hex filename
  54993   17248     0  72241  11a31 ./drivers/net/ethernet/ti/cpsw_new.o

After:
   text    data   bss    dec    hex filename
  54784   17376     0  72160  119e0 ./drivers/net/ethernet/ti/cpsw_new.o

(gcc version 10.3.0)

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