Bluetooth: Remove special handling of ATT in l2cap_security_cfm()

With the update to sk->resume() and __l2cap_no_conn_pending() we
no-longer need to have special handling of ATT channels in the
l2cap_security_cfm() function. The chan->sec_level update when
encryption has been enabled is safe to do for any kind of channel, and
the loop takes later care of calling chan->ready() or chan->resume() if
necessary.

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