cxgb4: hide unused warnings

The two new variables are only used inside of an #ifdef and cause
harmless warnings when that is disabled:

drivers/net/ethernet/chelsio/cxgb4/cxgb4_main.c: In function 'init_one':
drivers/net/ethernet/chelsio/cxgb4/cxgb4_main.c:4646:9: error: unused variable 'port_vec' [-Werror=unused-variable]
drivers/net/ethernet/chelsio/cxgb4/cxgb4_main.c:4646:6: error: unused variable 'v' [-Werror=unused-variable]

This adds another #ifdef around the declarations.

Fixes: 96fe11f2 ("cxgb4: Implement ndo_get_phys_port_id for mgmt dev")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>