macb: fix build warning for !CONFIG_OF

When CONFIG_OF is disabled, we get a harmless warning about the
newly added variable:

drivers/net/ethernet/cadence/macb_main.c:48:39: error: 'mgmt' defined but not used [-Werror=unused-variable]
 static struct sifive_fu540_macb_mgmt *mgmt;

Move the variable closer to its use inside of the #ifdef.

Fixes: c218ad55 ("macb: Add support for SiFive FU540-C000")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>