dmaengine: stm32-dmamux: Fix comment typo

The double `end' is duplicated in the comment, remove one.

Signed-off-by: Jason Wang <wangborong@cdjrlc.com>
Link: https://lore.kernel.org/r/20220811120959.18752-1-wangborong@cdjrlc.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>