Bluetooth: Mesh: Use dfd_phase_set() in dfu_suspended()

Currently, dfu_suspended() sets the phase SUSPENDED
directly in the structure, bypassing the dfd_phase_set() function.
This prevents the phase change callback in the bt_mesh_dfd_srv_cb
structure from receiving the SUSPENDED event.

Signed-off-by: Emilio Aguila Escalante <emilio.aguila@hotmail.com>