Bluetooth: Allow only one LE connection attempt

Only one outgoing LE connection attempt should be possible.
hci_connect() will now return -EBUSY in case there's another pending
outgoing connection.

Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@tieto.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>