usb/isp1760: Move function isp1760_endpoint_disable() within file.

Preparation for patch #2. The function isp1760_endpoint_disable() does almost
the same thing as urb_dequeue(). In patch #2 I change these to use a common
helper function instead of calling each other - for clarity but also to
avoid releasing the spinlock while in a "questionable" state. It seemed
proper to have these functions close to each other in the code.

Signed-off-by: Arvid Brodin <arvid.brodin@enea.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>