Bluetooth: Add support for waiting for passkey request

When our side is the acceptor of a passkey entry based SC pairing and
the remote side performs passkey notification instead of passkey entry,
we will receive the first pairing confirm PDU before we know what the
passkey is.

This patch adds a new SMP flag to indicate that we are waiting for the
passkey and reuses the existing CFM_PENDING flag to indicate that we've
received a pairing confirm PDU but not yet reacted to it. Once we
finally receive the passkey we check these flags and proceed with the
pairing as appropriate.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>