wlcore: invalidate keep-alive template on disconnection

Previously, invalidation of the keep-alive template was
done when going idle. However, while removing the
idle-handling we didn't move the keep-alive template
invalidation to another place.

This finally resulted in fw error when trying to use
the keep-alive template by another role.

(Note that we still have an error here - each role
should have its unique keep-alive template id, while
currently they all use CMD_TEMPL_KLV_IDX_NULL_DATA (0).
This only works now because we don't support concurrent
connected stations yet)

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <luca@coelho.fi>