[libata] pata_serverworks: fill in ->irq_clear hook

Required by libata, as it is called unconditionally.

Fixes an obvious oops.
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/ata/pata_serverworks.c

@@ -356,6 +356,8 @@ static struct ata_port_operations serverworks_osb4_port_ops = {
 	.data_xfer	= ata_pio_data_xfer,
 
 	.irq_handler	= ata_interrupt,
+	.irq_clear	= ata_bmdma_irq_clear,
+
 	.port_start	= ata_port_start,
 	.port_stop	= ata_port_stop,
 	.host_stop	= ata_host_stop
@@ -389,6 +391,8 @@ static struct ata_port_operations serverworks_csb_port_ops = {
 	.data_xfer	= ata_pio_data_xfer,
 
 	.irq_handler	= ata_interrupt,
+	.irq_clear	= ata_bmdma_irq_clear,
+
 	.port_start	= ata_port_start,
 	.port_stop	= ata_port_stop,
 	.host_stop	= ata_host_stop