dma: imx-dma: Remove redundant NULL check

kfree on a NULL pointer is a no-op. Null pointer check is
not necessary.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>