target-mips: fix compiler warnings (clang 5)

static code analyzer complain:

target/mips/helper.c:453:5: warning: Function call argument is an uninitialized value
    qemu_log_mask(CPU_LOG_MMU,
    ^~~~~~~~~~~~~~~~~~~~~~~~~~

'physical' and 'prot' are uninitialized if 'ret' is not TLBRET_MATCH.

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>