dmaengine: at_xdmac: fix macro typo

Fix typo in a macro which was not used until now. It explains why there
is no error at compilation time.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Fixes: e1f7c9ee

 "dmaengine: at_xdmac: creation of the atmel eXtended
DMA Controller driver"
Cc: stable@vger.kernel.org # 3.19 and later
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>