usb: early: xhci-dbc: Fix a potential out-of-bound memory access

In the Linux kernel, the following vulnerability has been resolved: usb: early: xhci-dbc: Fix a potential out-of-bound memory access If xdbc_bulk_write() fails, the values in 'buf' can be anything. So the string is not guaranteed to be NULL terminated when xdbc_trace() is called. Reserve an extra byte, which will be zeroed automatically because 'buf' is a static variable, in order to avoid troubles, should it happen.