spi/dw_spi: refine the IRQ mode working flow

Now dw_spi core fully supports 3 transfer modes: pure polling,
DMA and IRQ mode. IRQ mode will use the FIFO half empty as
the IRQ trigger, so each interface driver need set the fifo_len,
so that core driver can handle it properly

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
diff --git a/drivers/spi/dw_spi_pci.c b/drivers/spi/dw_spi_pci.c
index 7980f14..1f0735f 100644
--- a/drivers/spi/dw_spi_pci.c
+++ b/drivers/spi/dw_spi_pci.c
@@ -73,6 +73,7 @@
 	dws->num_cs = 4;
 	dws->max_freq = 25000000;	/* for Moorestwon */
 	dws->irq = pdev->irq;
+	dws->fifo_len = 40;		/* FIFO has 40 words buffer */
 
 	ret = dw_spi_add_host(dws);
 	if (ret)