Bluetooth: btwilink: Fix probe return value

Probe functions should return 0 on success. This driver's probe
returns the value returned by hci_register_dev(), which is the hci
index. This works for systems with only one hci device (id = 0) but
for systems where the btwilink device ends up with an id larger than
0, things will start to fall apart.

Make the probe function return 0 on success.

Signed-off-by: Jacob Siverskog <jacob@teenage.engineering>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>