Bluetooth: Use the MEDIUM security level for pairings

This lifts the requirement of 16 digits pin codes when pairing
with devices that do not support SSP when using the mgmt interface.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>