target/ppc: replaced the TODO with LOG_UNIMP and add break for silence warnings

target/ppc: replaced the TODO with LOG_UNIMP and add break for silence warnings

When using -Wimplicit-fallthrough in our CFLAGS, the compiler showed warning:
target/ppc/mmu_helper.c: In function ‘dump_mmu’:
target/ppc/mmu_helper.c:1351:12: warning: this statement may fall through [-Wimplicit-fallthrough=]
 1351 |         if (ppc64_v3_radix(env_archcpu(env))) {
      |            ^
target/ppc/mmu_helper.c:1358:5: note: here
 1358 |     default:
      |     ^~~~~~~

Use "qemu_log_mask(LOG_UNIMP**)" instead of the TODO comment.
And add the break statement to fix it.

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20201116024810.2415819-8-kuhn.chenqun@huawei.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>