target/mips: Replace gen_exception_err(err=0) by gen_exception_end()

generate_exception_err(err=0) is simply generate_exception_end().

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201214183739.500368-11-f4bug@amsat.org>