spi: spi-ti-qspi: Fix a bug when accessing non default CS

spi: spi-ti-qspi: Fix a bug when accessing non default CS

When switching ChipSelect from default CS0 to any other CS, driver fails
to update the bits in system control module register that control which
CS is mapped for MMIO access. This causes reads to fail when driver
tries to access QSPI flash on CS1/2/3.

Fix this by updating appropriate bits whenever active CS changes.

Reported-by: Andreas Dannenberg <dannenberg@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lore.kernel.org/r/20191211155216.30212-1-vigneshr@ti.com
Signed-off-by: Mark Brown <broonie@kernel.org>