spi/dw_spi: enable platform specific chipselect.

The driver core allows for a platform-specific chipselect assert/deassert
function, however the chipselect function in the core doesn't take advantage
of this fact.

This enables the use of a custom function, should it be defined.

Signed-off-by: George Shore <george@georgeshore.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

include/linux/spi/dw_spi.h

@@ -172,6 +172,10 @@ static inline void spi_chip_sel(struct dw_spi *dws, u16 cs)
 {
 	if (cs > dws->num_cs)
 		return;
+
+	if (dws->cs_control)
+		dws->cs_control(1);
+
 	dw_writel(dws, ser, 1 << cs);
 }