ravb: make ravb_ptp_interrupt() *void*

When we have the ISS.CGIS bit set, we already know that gPTP interrupt has
happened, so an extra GIS register check at the end of ravb_ptp_interrupt()
seems superfluous.  We can model the gPTP interrupt  handler like all other
dedicated interrupt handlers in the driver and make it *void*.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>