net: dsa: mv88e6xxx: fix freeing unused SERDES IRQ

Now mv88e6xxx does not enable its ports at setup itself and let
the DSA core handle this, unused ports are disabled without being
powered on first. While that is expected, the SERDES powering code
was assuming that a port was already set up before powering it down,
resulting in freeing an unused IRQ. The patch fixes this assumption.

Fixes: b759f528 ("net: dsa: mv88e6xxx: enable SERDES after setup")
Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>