Bluetooth: Fix error response for simultaneous fast connectable commands

If there's another pending mgmt_set_fast_connectable command we should
return a "busy" error response.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>