Bluetooth: Mesh: Fix friendship credential usage for segmented messages

The friend_cred hint needs to be set already at the point of
segmenting, i.e. doing it in bt_mesh_net_send() is too late. Move the
setting to bt_mesh_trans_send() and bt_mesh_ctl_send().

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>