net: phy: dp83867: fix irq generation

For proper IRQ generation by DP83867 phy the INT/PWDN pin has to be
programmed as an interrupt output instead of a Powerdown input in
Configuration Register 3 (CFG3), Address 0x001E, bit 7 INT_OE = 1. The
current driver doesn't do this and as result IRQs will not be generated by
DP83867 phy even if they are properly configured in DT.

Hence, fix IRQ generation by properly configuring CFG3.INT_OE bit and
ensure that Link Status Change (LINK_STATUS_CHNG_INT) and Auto-Negotiation
Complete (AUTONEG_COMP_INT) interrupt are enabled. After this the DP83867
driver will work properly in interrupt enabled mode.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>