Bluetooth: Make SMP context private to smp.c

There are no users of the smp_chan struct outside of smp.c so move it
away from smp.h. The addition of the l2cap.h include to hci_core.c,
hci_conn.c and mgmt.c is something that should have been there already
previously to avoid warnings of undeclared struct l2cap_conn, but the
compiler warning was apparently shadowed away by the mention of
l2cap_conn in the struct smp_chan definition.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>