USB: sierra: minimise no-suspend window during close

Move usb_autopm_get_interface_no_resume to the end of close(). This
makes the window during which suspend is prevented before the final put
in USB serial core slightly smaller.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>