Bluetooth: Introduce hci_conn_background_le_scan

This patch introduces the hci_conn_background_le_scan function which
starts the background passive scanning we have pending LE connections.
In case of HCI command failure (the controller couldn't enable the LE
scanning), all pending LE connections terminated.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Bluetooth: Background LE scanning

In order to be able to connection to devices as soon as the are
in-range, we should keep scanning. Therefore, we should start the
background scanning when the Device discovery stops or when an LE
connection attempt terminates.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Bluetooth: Remove BT_CONNECT check in hci_connect_le

Since the we supported only on LE connection attempt at a time, we
had to check if we already had one connection attempt running.
However, now we support multiple LE connection attempts so we can
safely remove this check in hci_connect_le function.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>