There is no user of few headers without CONFIG_SPI_DW_MID_DMA being set.
Move them under condition.

While at it, remove unused slab.h there.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200506153025.21441-5-andriy.shevchenko@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>