spi: dw: Move few headers under #ifdef CONFIG_SPI_DW_MID_DMA

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>

drivers/spi/spi-dw-mid.c

@@ -5,15 +5,14 @@
  * Copyright (c) 2009, 2014 Intel Corporation.
  */
 
-#include <linux/dma-mapping.h>
-#include <linux/dmaengine.h>
-#include <linux/slab.h>
 #include <linux/spi/spi.h>
 #include <linux/types.h>
 
 #include "spi-dw.h"
 
 #ifdef CONFIG_SPI_DW_MID_DMA
+#include <linux/dma-mapping.h>
+#include <linux/dmaengine.h>
 #include <linux/irqreturn.h>
 #include <linux/pci.h>
 #include <linux/platform_data/dma-dw.h>