ASoC: sn95031: Replace direct snd_soc_codec dapm field access

ASoC: sn95031: Replace direct snd_soc_codec dapm field access

The dapm field of the snd_soc_codec struct is eventually going to be
removed, in preparation for this replace all manual access to
codec->dapm.bias_level with snd_soc_codec_get_bias_level().

While we are at it also replace the if(x == A) ... else if(x == B) ...
construct with a switch-case.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>