mei: fix regressions caused by removing ext_msg

1. Delete cb from list before freeing it
2. Fix missed break that leads to
switch case fall-through and BUG invocation.

Regression from:
commit 6bb948c9
mei: get rid of ext_msg

Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>