dma40: ensure event lines get enabled

The controller sometimes fails to register the enable of the event line when
both src and dst event lines are used on the same logical channel.  Implement
the recommended software workaround, which is to retry the write until it
works.

Acked-by: Per Forlin <per.forlin@stericsson.com>
Acked-by: Jonas Aaberg <jonas.aberg@stericsson.com>
Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>