Bluetooth: Mesh: fix start rx transaction before tx is ended

Commit fixes behavior when provisioning protocol accepts
incoming transaction before ongoing tx transaction is
still active (device didn't receive acknowledgement yet).
Incoming data are ignored until ack is not received.

Specification:
5.3.3 Generic Provisioning behavior
...
If the sender receives a Transaction Acknowledgement
message, then the transaction has completed.
If the sender receives a message with other PDU types,
then the message shall be ignored.
...

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>