slimbus: Re-organize the mutex lock for reconfigure_now api

Re-arange the mutex lock for reconfigure_now api to avoid the
race conditions in slimbus channel state access.

Change-Id: I74743c93fafecac88767a2baa825c080c970b886
Signed-off-by: Prudhvi Yarlagadda <pyarlaga@codeaurora.org>