ASoC: wm9081: fixup wm9081_digital_mute() reg read

commit 48c33876 ("ASoC: wm9081: replace codec to component")
replaced codec to component, but it didn't keep WM9081_DAC_DIGITAL_2
value to reg. This patch fixup it.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>