spi: dw: Use an explicit set_cs assignment

Simplify the dw_spi_add_host() method a bit by replacing the currently
implemented default set_cs callback setting up and later having it
overwritten by a custom function with direct if-else-based callback
assignment.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20201007235511.4935-2-Sergey.Semin@baikalelectronics.ru


Signed-off-by: Mark Brown <broonie@kernel.org>