Bluetooth: Mesh: LPN non-low power establishment set state

Sets the LPN state to WAIT_UPDATE when waiting for the initial poll
response when disabling LPN_ESTABLISHMENT. Previously, the LPN node
would stay in the wait offer state even after the offer was processed,
which led to it aborting the friend establishment completely if it
failed to receive the first poll.

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