smsc911x: register isr as IRQF_SHARED

The isr supports shared operation, so register it with the IRQF_SHARED
flag to indicate this.

This patch also removes the IRQF_DISABLED flag.  This driver doesn't
need it, and IRQF_DISABLED isn't guaranteed when using shared interrupts.

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