spi: zynq-qspi: Anything else than CS0 is not supported yet

Unlike what the driver is currently advertizing, CS0 only can be used,
CS1 is not supported at all. Prevent people to use CS1.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20191108140744.1734-2-miquel.raynal@bootlin.com


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