Bluetooth: Fix SM pairing parameters negotiation

Before implementing SM key distribution, the pairing features
exchange must be better negotiated, taking into account some
features of the host and connection requirements.

If we are in the "not pairable" state, it makes no sense to
exchange any key. This allows for simplification of the key
negociation method.

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