Bluetooth: Require authentication if MITM protection is requested

The HIGH security level requires a 16 digit pin code for non-SSP
bondings. Sometimes this requirement is not acceptable and we still
want protection againts MITM attacks (which is something that the
MEDIUM security level doesn't provide), for that we should allow
another way to request authentication without using the HIGH security
level.

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