Bluetooth: Remove locking in hci_user_passkey_request_evt

This patch removes hdev locking in hci_user_passkey_request_evt
since it is not needed. mgmt_user_passkey_request simply calls
mgmt_event which does not require hdev locking at all.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>