net: of_mdio: do not overwrite PHY interrupt configuration

If irq_of_parse_and_map fails to find an interrupt line for a given PHY,
we will force the PHY interrupt to be PHY_POLL, completely overriding
the previous value that the MDIO bus may have set for us (e.g:
PHY_IGNORE_INTERRUPT). In case of failure, just restore the previous
value.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>