fs: yaffs2: fix the wrong check of return value of dir_emit()

Fix the following build warning:
fs/yaffs2/yaffs_vfs.c:1788:34: warning: comparison of constant '0' with boolean expression is always false [-Wbool-compare]
           this_inode, this_type) < 0) {

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>