net: sxgbe: make "core_ops" static

The "core_ops" variable isn't referenced outside this file and Sparse
complains about it:

	drivers/net/ethernet/samsung/sxgbe/sxgbe_core.c:239:29: warning:
	symbol 'core_ops' was not declared. Should it be static?

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>