Bluetooth: Mesh: Fixes existing friend lookup in Friend Request handling

Currently, when handling a Friend Request message with `prev_addr` set,
we look up existing friend entry using `prev_addr` as the address.
However, `prev_addr` is the address of the requesting node's previous
friend, NOT the address of the requesting node itself. Therefore, we
should always look up existing friend entry using `rx->ctx.addr` as the
address.

Signed-off-by: Jiahao Li <reg@ljh.me>