soc: spcom: fix race during channel character dev creation

During char device creation, device counted by chdev_count and since
the device creation can happen from different contexts, need to guard the
flow.

This change introduces new lock for spcom_dev->chdev_count.

Change-Id: If59c3ae69d7366892583b2552cf276b6714eeb03
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>