spi: dw: fix kernel crash due to NULL pointer dereference

The obvious fix after the commit d9c73bb8a3a5 "spi: dw: add support for gpio
controlled chip select". This patch fixes the issue by using locally defined
temporary variable.

Fixes: d9c73bb8a3a5 (spi: dw: add support for gpio controlled chip select)
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: <stable@vger.kernel.org>