sim/rx: provide a format string for printf

sim/rx: provide a format string for printf

Calling printf with the format being a non constant string results in
a GCC warning:

  warning: format not a string literal and no format arguments [-Wformat-nonliteral]

Provide a constant format string to printf in the few places this
warning is triggered.

sim/rx/ChangeLog:

	* reg.c (fpsw2str): Provide a format string to printf.
	(trace_register_changes): Likewise.