Bluetooth: Add timeout for LE connection attempts

LE connection attempts do not have a controller side timeout in the same
way as BR/EDR has (in form of the page timeout). Since we always do
scanning before initiating connections the attempts are always expected
to succeed in some reasonable time.

This patch adds a timer which forces a cancellation of the connection
attempt within 20 seconds if it has not been successful by then. This
way we e.g. ensure that mgmt_pair_device times out eventually and gives
an error response.

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