ASoC: SSM2602: Fix 'Mic Boost2' control

The 'Mic Boost2' control's shift was off by one and thus was not working.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@kernel.org