DP_PTASK: Fix warning may be used uninitialized
    
    It could be unitialized only in case if we add new enum be_ptask_schedule
    Currently, we have only BE_PTASK_SCHEDULE_FROM_NOW and
    BE_PTASK_SCHEDULE_FROM_LAST which are properly covered in switch case.
    
    src/providers/dp_ptask.c:200: warning: ‘tv’ may be used uninitialized in this function
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    (cherry picked from commit 911e9879cae1f8f18e73acb3c6037af651cd9db5)