spi: dw: remove cs_control and poll_mode members from chip_data

Since these members were initialized only with previous dw_spi_chip
struct members and that there is no user anymore, remove them. Along
this removal, remove code path which were using these members.

Signed-off-by: Clement Leger <cleger@kalray.eu>
Link: https://lore.kernel.org/r/20200416110916.22633-2-cleger@kalray.eu
Signed-off-by: Mark Brown <broonie@kernel.org>