net: dsa: mv88e6xxx: Reset mv88e6393x force WD event bit

The force watchdog event bit is not cleared during SW reset in the
mv88e6393x switch. This is a different behavior compared to mv886390 which
clears the force WD event bit as advertised. This causes a force WD event
to be handled over and over again as the SW reset following the event never
clears the force WD event bit.

Explicitly clear the watchdog event register to 0 in irq_action when
handling an event to prevent the switch from sending continuous interrupts.
Marvell aren't aware of any other stuck bits apart from the force WD
bit.

Fixes: de776d0d ("net: dsa: mv88e6xxx: add support for mv88e6393x family"
Signed-off-by: Gustav Ekelund <gustaek@axis.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>