spi: dw-mid: avoid potential NULL dereference

When DMA descriptor allocation fails we should not try to assign any fields in
the bad descriptor. The patch adds the necessary checks for that.

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>
Cc: stable@vger.kernel.org