net: dsa: mv88e6xxx: Fix serdes irq setup going recursive

Duec to a typo, mv88e6390_serdes_irq_setup() calls itself, rather than
mv88e6390x_serdes_irq_setup(). It then blows the stack, and shortly
after the machine blows up.

Fixes: 2defda1f ("net: dsa: mv88e6xxx: Add support for SERDES on ports 2-8 for 6390X")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>