net: dsa: mv88e6xxx: fix SERDES IRQ mapping

The current mv88e6xxx SERDES code checks for negative error code from
irq_find_mapping, while this function returns an unsigned integer. This
patch removes this dead code and simply returns 0 is no IRQ is found.

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