ASoC: nau8810: fix compile warning in loopback switch control

Thank Stephen Rothwell for the message.
The patch is to fix the following compile error.

sound/soc/codecs/nau8810.c:441:3: error: initialization from incompatible
pointer type [-Werror=incompatible-pointer-types]
SND_SOC_DAPM_SWITCH("Digital Loopback", SND_SOC_NOPM, 0, 0,
&nau8810_loopback),
^
sound/soc/codecs/nau8810.c:441:3: note: (near initialization for
'nau8810_dapm_widgets[11].kcontrol_news')

Signed-off-by: John Hsu <KCHSU0@nuvoton.com>
Signed-off-by: Mark Brown <broonie@kernel.org>