rt2x00usb: initialize the read value in case of failure

My understanding ist that rt2x00usb_register_read() is void and so the
reader is unaware of read errors and assumes that whatever was on the
stack as it was about to read.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>