MIPS: mips32: fix wrong loop bound in flush_cache()

The issue is found when calling flush_cache() with zero "size" argument.
The bound of loop is miscalculated in this case and flush_cache() enters
a wrong flushing loop.

Signed-off-by: Yao Cheng <saturdaycoder@gmail.com>
Cc: Shinya Kuribayashi <skuribay@pobox.com>
Cc: Sergei Shtylyov <sshtylyov@mvista.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Shinya Kuribayashi <skuribay@pobox.com>