Bluetooth: Fix using HCI_CONN_LE_SMP_PEND to check for SMP context

The code is consistently using the HCI_CONN_LE_SMP_PEND flag check for
the existence of the SMP context, with the exception of this one place
in smp_sig_channel(). This patch converts the place to use the flag just
like all other instances.

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