Bluetooth: Use defines in in hci_get_auth_req()

Make the code in hci_get_auth_req() more readable by using the
defined macros instead of inlining magic numbers.

Signed-off-by: Mikel Astiz <mikel.astiz@bmw-carit.de>
Signed-off-by: Timo Mueller <timo.mueller@bmw-carit.de>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>