spi: bcm-qspi: Convert to use CS GPIO descriptors

Set use_gpio_descriptors to true and avoid asserting the native chip
select if the spi core has done it for us.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Link: https://lore.kernel.org/r/20191107044235.4864-2-chris.packham@alliedtelesis.co.nz


Signed-off-by: Mark Brown <broonie@kernel.org>