smsc911x: add support for platform-specific irq flags

this patch adds support for the platform_device's resources to indicate
additional flags to use when registering the irq, for example
IORESOURCE_IRQ_LOWLEVEL (which corresponds to IRQF_TRIGGER_LOW).  These
should be set in the irq resource flags field.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>