dma/imx-sdma: use num_events to validate event_id0

event number is not always 32. use num_events for checking instead.

Signed-off-by: Richard Zhao <richard.zhao@linaro.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>