usb: chipidea: debug: avoid out of bound read

A string written by the user may not be zero terminated.

sscanf may read memory beyond the buffer if no zero byte
is found.

For testing build with CONFIG_USB_CHIPIDEA=y, CONFIG_USB_CHIPIDEA_DEBUG=y.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Peter Chen <peter.chen@freescale.com>