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>

include/linux/spi/dw_spi.h

@@ -90,6 +90,7 @@ struct dw_spi {
 	unsigned long		paddr;
 	u32			iolen;
 	int			irq;
+	u32			fifo_len;	/* depth of the FIFO buffer */
 	u32			max_freq;	/* max bus freq supported */
 
 	u16			bus_num;