Bluetooth: btusb: be quiet on device disconnect

Disabling the bluetooth usb device embedded in (some) ThinkPads tends to
lead to errors like these:
    btusb_bulk_complete: hci0 urb ffff88011b9bfd68 failed to resubmit (19)
    btusb_intr_complete: hci0 urb ffff88011b46a318 failed to resubmit (19)
    btusb_bulk_complete: hci0 urb ffff88011b46a000 failed to resubmit (19)

That is because usb_disconnect() doesn't "quiesces" pending urbs.

Disconnecting a device is a normal thing to happen so it's no big deal
that usb_submit_urb() returns -ENODEV. The simplest way to get rid of
these errors is to stop treating that return as an error. Trivial,
actually.

While we're at it, add comments to be explicit about the reasons we're
not complaining about -EPERM and -ENODEV.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>