net: dsa: mv88e6xxx: centralize SERDES IRQ handling

The .serdes_irq_setup are all following the same steps: get the SERDES
lane, get the IRQ mapping, request the IRQ, then enable it. So do
the .serdes_irq_free implementations: get the SERDES lane, disable
the IRQ, then free it.

This patch removes these operations in favor of generic functions.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>