dmaengine: at_hdmac: remove channel status testing in tasklet

There is no need to test if channel is enabled in tasklet:
- in error path, channel is disabled in interrupt routine
- in normal path, this test is performed in sub functions to report
a misuse of the engine.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>