After the previous cleanup, there is no user of send_mbox_cmd outside of
isst-core-mbox.c.
Thus move send_mbox_cmd to isst-core-mbox.c as internal functions.

No functional changes are expected.
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
[srinivas.pandruvada@linux.intel.com: changelog edits]
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>