ASoC: codecs: PCM1789: unconditionally flush work

Work is guaranteed to be initialized on exit. Drop the unnecessary
if statement and always call flush_work.

This fixes a warning seen with clang:
sound/soc/codecs/pcm1789.c:265:13: warning: address of 'priv->work' will
      always evaluate to 'true' [-Wpointer-bool-conversion]
        if (&priv->work)
        ~~   ~~~~~~^~~~

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Mark Brown <broonie@kernel.org>