target/mips: Tidy control flow in mips_cpu_handle_mmu_fault

target/mips: Tidy control flow in mips_cpu_handle_mmu_fault

Since the only non-negative TLBRET_* value is TLBRET_MATCH,
the subsequent test for ret < 0 is useless.  Use early return
to allow subsequent blocks to be unindented.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>