Bluetooth: Add passkey entry support for LE SC

Bluetooth: Add passkey entry support for LE SC

The passkey entry mechanism involves either both sides requesting the
user for a passkey, or one side requesting the passkey while the other
one displays it. The behavior as far as SMP PDUs are concerned are
considerably different from numeric comparison and therefore requires
several new functions to handle it.

In essence passkey entry involves both sides gradually committing to
each bit of the passkey which involves 20 rounds of pairing confirm and
pairing random PDUS being sent in both directions.

This patch adds a new smp->passkey_round variable to track the current
round of the passkey commitment and reuses the variables already present
in struct hci_conn for the passkey and entered key count.

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