ASoC: ad1980: Fix wrong number of items for capture source

The number of capture sources is 8, not 7.
Use SOC_ENUM_DOUBLE_DECL() macro and it's automatically fixed.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>