Bluetooth: Set security level on incoming pairing request

If a master would like to raise the security level, it will send a
pairing request. While the pending security level is set on an incoming
security request (from a slave), it is not set on a pairing request. As
a result, the security level would not be raised on the slave in such
case.

Fix this by setting the pending security when receiving pairing
requests according to the requested authorization.

Signed-off-by: Ido Yariv <ido@wizery.com>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>