spi-gpio: init CS before spi_bitbang_setup()

spi_bitbang_setup() deasserts the chip select line to initialise
the device. The chip select GPIO line is obtained from
spi_gpio->cs_gpios[] private data.
Currently, devices that are not registered under devicetree
environment will call into spi_bitbang_setup() with stale
cs_gpios[].

This patch ensures spi_gpio->cs_gpios[] is always initialised prior
to calling spi_bitbang_setup().

Reviewed-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Josef Ahmad <josef.ahmad@intel.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>