spi: mchp-pci1xxxx: Fix improper implementation of disabling chip select lines

spi: mchp-pci1xxxx: Fix improper implementation of disabling chip select lines

Hardware does not have support to disable individual chip select lines.
Disable all chip select lines by using SPI_FORCE_CE bit.

Fixes: 1cc0cbea7167 ("spi: microchip: pci1xxxx: Add driver for SPI controller of PCI1XXXX PCIe switch")
Signed-off-by: Tharun Kumar P <tharunkumar.pasumarthi@microchip.com>
Link: https://lore.kernel.org/r/20230404171613.1336093-4-tharunkumar.pasumarthi@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>