spi: dw-mid: check that DMA was inited before exit

If the driver was compiled with DMA support, but DMA channels weren't acquired
by some reason, mid_spi_dma_exit() will crash the kernel.

Fixes: 7063c0d9 (spi/dw_spi: add DMA support)
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>