USB: usb-skeleton: fix runtime PM after driver unbind

Since commit c2b71462 ("USB: core: Fix bug caused by duplicate
interface PM usage counter") USB drivers must always balance their
runtime PM gets and puts, including when the driver has already been
unbound from the interface.

Leaving the interface with a positive PM usage counter would prevent a
later bound driver from suspending the device.

Fixes: c2b71462

 ("USB: core: Fix bug caused by duplicate interface PM usage counter")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191001084908.2003-2-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>