phylib: Add interrupt source check function to M88E1121R driver

Add did_interrupt() function to check if a PHY port
really caused an interrupt. This is needed in the case
of shared PHY interrupt pin configuration to stop
interrupt event processing for PHY ports which didn't
cause an interrupt.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>