net: mscc: ocelot: Fix uninitialized error in ocelot_netdevice_event()

With gcc-4.1.2:

    drivers/net/ethernet/mscc/ocelot.c: In function ‘ocelot_netdevice_event’:
    drivers/net/ethernet/mscc/ocelot.c:1129: warning: ‘ret’ may be used uninitialized in this function

If the list iterated over by netdev_for_each_lower_dev() is empty, ret
is never initialized, and converted into a notifier return value.

Fix this by preinitializing ret to zero.

Fixes: a556c76a

 ("net: mscc: Add initial Ocelot switch support")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>