spi: dw: Remove unused members from struct chip_data

Local struct chip_data has two members that are not used:

- cs. Looks like was never used
- enable_dma. Became unused by the commit f89a6d8f ("spi: dw-mid: move
  to use core SPI DMA mappings").

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>