[undi] Assume that interrupts are not supported if IRQ=0

Some PXE stacks (notably old Etherboot/gPXE stacks) will claim to use
the timer interrupt, rather than reporting that interrupts are not
supported.  Since using the timer interrupt is equivalent to polling
anyway, we may as well genuinely poll these stacks.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/drivers/net/undinet.c

 	DBGC ( undinic, "UNDINIC %p has type %s, speed %d, flags %08x\n",
 	       undinic, undi_iface.IfaceType, undi_iface.LinkSpeed,
 	       undi_iface.ServiceFlags );
-	if ( undi_iface.ServiceFlags & SUPPORTED_IRQ )
+	if ( ( undi_iface.ServiceFlags & SUPPORTED_IRQ ) &&
+	     ( undinic->irq != 0 ) ) {
 		undinic->irq_supported = 1;
+	}
 	DBGC ( undinic, "UNDINIC %p using %s mode\n", undinic,
 	       ( undinic->irq_supported ? "interrupt" : "polling" ) );
 	if ( strncmp ( ( ( char * ) undi_iface.IfaceType ), "Etherboot",