bluetooth: host: fix unpacked l2cap struct

The bt_l2cap_le_conn_rsp struct in l2cap_internal.h has not been
declared __packed. This can cause alignment problems on some
platforms if the struct is placed on an unaligned address.

A __packed declaration solves this issue by forcing the compiler to
use store instructions that do not required alignment.

Signed-off-by: Jim Luther <jilu@oticon.com>