drivers: net: vsc9953: Fix FDB aging time

By default, the aging period is set to 0, so the dynamic
FDB entries are never removed. This patch sets the aging
time to 300 seconds.

Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@freescale.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>