Fix HID class device driver -- if a SetIDle request is issued with the LSB of wValue...

Fix Keyboard and Mouse demos, Idle period is now multiplied by 4 as the period is read into and sent out of the device to ensure it is always stored as a multiple of 1ms. Fixes Keyboard demo using an initial Idle period of 2s rather than 500ms (thanks to Brian Dickman).

Move out the internal device serial descriptor reading routine into a seperate static function, rather than being part of USB_Device_GetDescriptor.