Bluetooth: host: Fix crash in bt_l2cap_chan_recv_complete

It is possible that LE CoC channel is in disconnecting state (eg due
to peer sending too many packets) but application is not yet aware
of this (ie disconnected callback was not called) and thus may call
bt_l2cap_chan_recv_complete() to return credits. In this case it
leads to assert in l2cap_chan_send_credits.

It looks like PTS 8.2.1 is able to trigger this scenario when
executing L2CAP/ECFC/BI-02-C test.

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>