[Yaffs] Fix in yaffs/direct/yaffsfs.c

Hi All

I found that there was a problem in the YAFFS/direct device look up which
could cause the wrong device to be matched.

If the device list contains a device with a path prefix that is a substring
of another (eg. /flash" and "/flash1", then the wrong device could be found
(ie. could find "/flash" instead of "/flash1").

This has been fixed.

-- Charles