dmaengine: edma: Check the current decriptor first in tx_status()

It's likely that the caller investigates the status of a currently
active descriptor. Make that simple check first and only rumage in the
vchan list if that fails.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Joel Fernandes <joelf@ti.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>