target-mips: replace break by goto cp0_unimplemented

this fixes many warnings like:

target/mips/translate.c:6253:13: warning: Value stored to 'rn' is never read
            rn = "invalid sel";
            ^    ~~~~~~~~~~~~~

Reported-by: Clang Static Analyzer
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Yongbok Kim <yongbok.kim@imgtec.com>
Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>