Bluetooth: Restore accidentally-deleted line

When code was moved from l2cap_core.c to l2cap_sock.c in commit
6de0702b, one line was dropped
from the old __l2cap_sock_close() implementation. This sk_state
change should still be in l2cap_chan_close().

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>