ASoC: sound: soc: fix incorrect max value

1. Max value stands for number of levels in SX control. The value
was adjusted inappropriately, which causes inconsistency between
max and min value.

2. Code assumes negative value of mc->min. Positive mc->min leads
to incorrect number of levels. The fix covers the risk.

CRs-Fixed: 1053735
Change-Id: I71bd8f11ff9faff9252129d84c1a80845fc2fe2a
Signed-off-by: Xiaojun Sang <xsang@codeaurora.org>