dmaengine: imx-sdma: Fix error checking in sdma_event_remap

of_parse_phandle() returns NULL on errors, rather than error
pointers. Using NULL check on grp_np to fix this.

Fixes: d078cd1b

 ("dmaengine: imx-sdma: Add imx6sx platform support")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Link: https://lore.kernel.org/r/20220308064952.15743-1-linmq006@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>