ASoC: wm8960: correct the min gain value of some PGA

The min gain is the corresponding gain value when the register value is 0
instead of 1, just correct it.

Signed-off-by: Zidan Wang <zidan.wang@freescale.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>