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

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

commit 63d84b1a46957b3ecc56af8087220c0df42e0ef2 upstream.

commit fb57862ead652454ceeb659617404c5f13bc34b5 upstream.

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: 7063c0d942a1 (spi/dw_spi: add DMA support)
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>