Fix directory look up.

Thanks to Trent Lillehaugen for new algorithm:

Let's say I have two yaffs partitions: "/" and "/foo".
If I call, yaffs_mkdir("/foobar", 0), yaffsfs_FindDevice will match the path
with the "/foo" partition.  I would expect it to match "/" instead.
It will also match "/fo/o/bar" with "/foo", again I think it should match
with "/".

Signed-off-by: Charles Manning <cdhmanning@gmail.com>