net: dsa: mv88e6xxx: wrap SERDES IRQ in power function

Now that mv88e6xxx_serdes_power is only called after driver setup,
we can wrap the SERDES IRQ code directly within it for clarity.

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