spi: dw: Set GPIO_SS flag to toggle Slave Select on GPIO CS

The Designware SPI master requires slave selection before the transfer
can begin [1].

This patch uses the new master flag to indicate both the GPIO CS and
the internal chip select should be used.

Tested On:
    Altera CycloneV development kit
    Compile tested for build errors on x86_64 (allyesconfigs)

[1] DesignWare dw_apb_ssi Databook, Version 3.20a (page 39)
Signed-off-by: Thor Thayer <tthayer@opensource.altera.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-dw.c

@@ -502,6 +502,7 @@ int dw_spi_add_host(struct device *dev, struct dw_spi *dws)
 	master->handle_err = dw_spi_handle_err;
 	master->max_speed_hz = dws->max_freq;
 	master->dev.of_node = dev->of_node;
+	master->flags = SPI_MASTER_GPIO_SS;
 
 	/* Basic HW init */
 	spi_hw_init(dev, dws);