net: dsa: bcm_sf2: share MDIO interrupts with MDIO bus driver

In preparation for switching all MDIO accesses to interrupt mode,
instead of polling mode, enable the MDIO done and error interrupts,
request the switch_0 interrupt line with IRQF_SHARED, and return
IRQ_NONE for the MDIO interrupts bits since we delegate them.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>