Bluetooth: Mesh: Fix recovery of WAITING_FOR_START BLOB Server state

If device running BLOB Server called `bt_mesh_blob_srv_recv`,
but rebooted before it received `XFER_START` message from BLOB Client,
it was wrongly "recovered" into Suspended phase, which would lead to
Server try to resume transfer on `XFER_START`. It would not be possible,
because `srv->state.xfer` was not set with acual values yet.

Set phase again to BT_MESH_BLOB_XFER_PHASE_WAITING_FOR_START, which will
allow to accept new transfer.

Signed-off-by: Krzysztof Kopyściński <krzysztof.kopyscinski@codecoup.pl>