Bluetooth: Refactor code for outgoing dedicated bonding

Do not always set the MITM protection requirement by default in the
field conn->auth_type, since this will be added later in
hci_io_capa_request_evt(), as part of the requirements specified in
HCI_OP_IO_CAPABILITY_REPLY.

This avoids a hackish exception for the auto-reject case, but doesn't
change the behavior of the code at all.

Signed-off-by: Mikel Astiz <mikel.astiz@bmw-carit.de>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>