target/mips: Remove spurious LOG_UNIMP of MTHC0 opcode

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>

target/mips/translate.c

@@ -5945,6 +5945,7 @@ static void gen_mthc0(DisasContext *ctx, TCGv arg, int reg, int sel)
         goto cp0_unimplemented;
     }
     trace_mips_translate_c0("mthc0", register_name, reg, sel);
+    return;
 cp0_unimplemented:
     qemu_log_mask(LOG_UNIMP, "mthc0 %s (reg %d sel %d)\n",