Added initialization of alarm severity on probe. This value is not us…

Created by: karlvestin

…ed, but should be initialized regardless. This was flagged as an error by the Codacy static code analysis.

Codacy link: https://app.codacy.com/gh/epics-base/epics-base/file/42103575016/issues/source?bid=16430872&fileBranchId=16430872#l604 Launchpad bug: https://bugs.launchpad.net/epics-base/+bug/1862918