Bluetooth: Avoid calling undefined smp_conn_security()

Commit ff03261adc8b4bdd8291f1783c079b53a892b429 ('Bluetooth: Fix
sending a HCI Authorization Request over LE links', commit
d8343f125710fb596f7a88cd756679f14f4e77b9 upstream) added an
call to smp_conn_security() from hci_conn_security().  The
former is only defined if CONFIG_BT_L2CAP=y.

This is not required in mainline since commit
f1e91e1640d808d332498a6b09b2bcd01462eff9 ('Bluetooth: Always compile
SCO and L2CAP in Bluetooth Core') removed that option.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>