dmaengine: imx-sdma: fix pagefault when channel is disabled during interrupt

Add a spinlock and a 'enabled' boolean on channel descriptor, to avoid
using buffer descriptors in the interrupt context,
when sdma_disable_channel is called meanwhile.

Signed-off-by: Thierry Bultel <tbultel@pixelsurmer.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>