ASoC: wm8962: 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() and replace all
other manual access to codec->dapm with snd_soc_codec_get_dapm().

Also drop the unnecessary comparison in the set_bias_level() callback that
checks if the device is already at the target level. The core already takes
care of this and will not call the callback if the device is already at the
target level.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>