tracing: Add TRACE_SYSTEM_VAR to xhci-hcd

New code will require TRACE_SYSTEM to be a valid C variable name,
but some tracepoints have TRACE_SYSTEM with '-' and not '_', so
it can not be used. Instead, add a TRACE_SYSTEM_VAR that can
give the tracing infrastructure a unique name for the trace system.

Cc: Xenia Ragiadakou <burzalodowa@gmail.com>
Cc: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Tested-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>