When running with '-d unimp' all MTHC0 opcode executed
are logged as unimplemented... Add the proper 'return'
statement missed from commit 5204ea79.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210422081055.2349216-1-f4bug@amsat.org>