ASoC: ak4671: Fix control-less DAPM routes

Routes without a control must use NULL for the control name. The ak4671
driver uses "NULL" instead in a few places. Previous to commit 5fe5b767
("ASoC: dapm: Do not pretend to support controls for non mixer/mux widgets")
the DAPM core silently ignored non-NULL controls on non-mixer and non-mux
routes. But starting with that commit it will complain and not add the
route breaking the ak4671 driver in the process.

This patch replaces the incorrect "NULL" control name with NULL to fix the
issue.

Fixes: 5fe5b767

 ("ASoC: dapm: Do not pretend to support controls for non mixer/mux widgets")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org