Bluetooth: Mesh: Check LPN credentials before establishment

While the friend node considers the friendship established as soon as it
sends the friendship offer, the LPN only considers the friendship
established once it receives the first update after its poll. This
update is encrypted with the friendship credentials, which aren't
available, as they're protected by an if (lpn_established()) check.

Changes the check to lpn_waiting_update(), which makes the LPN check its
friendship credentials only when it's expecting a response to a poll
message. This is the only time the friend will send messages encrypted
with the friendship credentials.

This is a regression from #28511.

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