bt: fix bt_l2cap_chan_send_sdu to return total bytes sent

bt_l2cap_chan_send_sdu previously returned the number of bytes sent
in the last sent sdu buf fragment or 0 if the buf has only
one fragment. bt_l2cap_chan_send_sdu now returns the total data
bytes sent from the buf.

Signed-off-by: Tom Finet <tom.codeninja@gmail.com>