Bluetooth: Mesh: Account for ASZMIC in encrypt

The decision to set aszmic = 1 in the net_tx structure was made after
the variable was passed to the crypto context, creating a discrepancy
between the two when aszmic is 1.

Extracts transport encryption to a separate function.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>