p54usb: stop USB core interference in exit path

The patch fixes a problem when the (Soft)LED stayed on after the module was unloaded.
It turned out that the USB core disables all endpoints before calling the disconnect method.
So it was impossible to switch off the radio & LEDs.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>