Bluetooth: Mesh: Fix sequence number in Friend queue

The code was updating net_rx->seq to make sure sdu_recv() gets the
right sequence number (seq_auth for segmented messages), however later
net_rx->seq was also used for enqueuing to the Friend Queue, causing
the queued messages to have the wrong value.

To fix this, don't update net_rx->seq, rather pass an explicit
sequence number value to sdu_recv(), which is just net_rx->seq for
unsegmented messages, and seq_auth for segmented messages.

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