sfc: Correct reporting and validation of TX interrupt coalescing

The reported TX IRQ moderation is generated in a completely crazy way.
Make it simple and correct.

When channels are shared between RX and TX, TX IRQ moderation must be
the same as RX IRQ moderation, but must be specified as 0!  Allow it
to be either specified as the same, or left at its previous value
in which case it will be quietly overridden.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ethernet/sfc/efx.h b/drivers/net/ethernet/sfc/efx.h
index 8f5acae..8ca6863 100644
--- a/drivers/net/ethernet/sfc/efx.h
+++ b/drivers/net/ethernet/sfc/efx.h
@@ -113,6 +113,8 @@
 extern void efx_schedule_reset(struct efx_nic *efx, enum reset_type type);
 extern void efx_init_irq_moderation(struct efx_nic *efx, unsigned int tx_usecs,
 				    unsigned int rx_usecs, bool rx_adaptive);
+extern void efx_get_irq_moderation(struct efx_nic *efx, unsigned int *tx_usecs,
+				   unsigned int *rx_usecs, bool *rx_adaptive);
 
 /* Dummy PHY ops for PHY drivers */
 extern int efx_port_dummy_op_int(struct efx_nic *efx);