spi: spi-gpio: Fix compiler warning when building for 64 bit systems

The assignment of SPI_GPIO_NO_CHIPSELECT to cs_gpios[0] causes the following
compiler warning, when building for 64 bit systems:
"warning: overflow in implicit constant conversion [-Woverflow]".

This is because the SPI_GPIO_NO_CHIPSELECT flag is a '-1' type casted to
unsigned long and cs_gpios is of the type int.

Furthermore the chip select's GPIO number is locally stored as unsigned int
and compared with SPI_GPIO_NO_CHIPSELECT. Thus the result of the comparison
is always false, if unsigned long and unsigned int have a different size.

As part of the fix this patch adds a check for the device tree's cs-gpios
property.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Torsten Fleischer <torfl6749@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>