Bluetooth: Make mgmt_stop_discovery_failed static

mgmt_stop_discovery_failed is now only used in mgmt.c so we can
make it a local function. This patch also moves the mgmt_stop_
discovery_failed definition up in mgmt.c to avoid forward
declaration.

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