spi: spi-microchip-core: Fix the number of chip selects supported

spi: spi-microchip-core: Fix the number of chip selects supported

The SPI "hard" controller in PolarFire SoC has eight CS lines, but only
one CS line is wired. When the 'num-cs' property is not specified in
the device tree, the driver defaults to the MAX_CS value, which has
been fixed to 1 to match the hardware configuration; however, when the
'num-cs' property is explicitly defined in the device tree, it
overrides the default value.

Fixes: 9ac8d17694b6 ("spi: add support for microchip fpga spi controllers")
Signed-off-by: Prajna Rajendra Kumar <prajna.rajendrakumar@microchip.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://msgid.link/r/20240514104508.938448-3-prajna.rajendrakumar@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>