net: dsa: mv88e6xxx: get serdes lane after lock

This is a follow-up patch for commit 17deaf5c ("net: dsa:
mv88e6xxx: create serdes_get_lane chip operation").

The .serdes_get_lane implementations access the CMODE of a port,
even though it is cached at the moment, it is safer to call them
after the mutex is locked, not before.

At the same time, check for an eventual error and return IRQ_DONE,
instead of blindly ignoring it.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>