Bluetooth: Refactor hci_get_auth_req()

Refactor the code without changing its behavior by handling the
no-bonding cases first followed by General Bonding.

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