[r8169] Disabling IRQs should not also acknowledge the IRQs

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

src/drivers/net/r8169.c

 
 	DBGP ( "rtl8169_irq_disable\n" );
 
-	rtl8169_irq_mask_and_ack ( ioaddr );
+	RTL_W16 ( IntrMask, 0x0000 );
 }
 
 /*** iPXE Core API Routines ***/