ASoC: tas2552: Use devm_gpiod_get_optional for enable-gpio

commit ea178d14

 ("ASoC: tas2552: Make the enable-gpio really optional")
makes enable-gpio optional. devm_gpiod_get_optional() is the better
function for optional gpio, so let's switch to use it.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>