Refactors retry loop in flush DAC work.  It is more clear to use a
for-loop here.

Uses !rt1015 to check if NULL.
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20201224100607.3006171-5-tzungbi@google.comSigned-off-by: Mark Brown <broonie@kernel.org>