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>