Bluetooth: Mesh: Do not transfer FW to targets in Verify Phase in DFU

These targets already received FW so there is no point to transfering
it again. What's more, these targets will not accept transfer and will
be dropped by DFU Client. If FW is already received DFU Client should
skip transfer and proceed to refresh procedure.

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