HUBPRE: Hub and low speed device

[tmk]

When USB Hub is connected to the converter HUBPRE bit is set.
https://github.com/tmk/USB_Host_Shield_2.0/blob/master/usbhub.cpp#L187

The HUBPRE bit is not cleared even after the hub is removed. This prevents low speed device from working when plugging it into the converter directly without the hub. (It works after replug twice though.)

The bit should be reset in Release()? Not sure this is correct but this patch solves the problem anyway.

tmk/tmk_keyboard#658

[tmk]

Fixed.
c28ad12