ASoC: tas2552: Remove unneeded NULL test for tas2552->enable_gpio

It's safe to call gpiod_set_value() with NULL desc.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>