spi: orion: Prevent incorrect chip select behaviour

spi: orion: Prevent incorrect chip select behaviour

When clearing the chip-select mask, the controller will switch to chip
selecting the native CS0 line. Because the control register chip-select
mask is not updated in a single write this will cause undesirable
chip-selection of CS0 even when requesting to select other native
chip-select lines. This is additionally problematic as the chip-select
may still be asserted. With the ARMADA 38x SoC the controller will
assert both the desired native chip-select and CS0.

To avoid any undesirable behaviour with the chip-select lines, update
the control register with a single write. This avoids selecting CS0 and
causes the (de-)assert to apply at the same time.

Signed-off-by: Nathan Rossi <nathan.rossi@digi.com>
Link: https://lore.kernel.org/r/20210816050228.3223661-1-nathan@nathanrossi.com
Signed-off-by: Mark Brown <broonie@kernel.org>