Bluetooth: Mesh: Ignore duplicate OOB upload request

If a Fw Distribution Client sends the Upload OOB Start message, but the
application layer didn't call bt_mesh_dfd_srv_oob_check_complete yet,
we have no other option other than ignore the message. The next phase
in this case could be Transfer Active, Transfer Success or Failed and it
will be set only after Check Firmware OOB procedure completes.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>