net: ti: prueth_switch: Ignore FDB table update when interface is down

When switch interfaces are down, PRUs are down and FDB table does not
exist. Therefore avoid accessing FDB table when interfaces are down.

Fixes: f3142191

 ("net: ti: prueth_core: Add support for RSTP switch")
Reported-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>