ASoC: pcm3168a: remove unused variable

While building we were getting build warning about:
sound/soc/codecs/pcm3168a.c:403:21: warning: variable 'channels' set but
not used

The variable channels were being assigned some value but that was never
reused.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Mark Brown <broonie@kernel.org>