grub: fs/fat: Don't error when mtime is 0

A issue was found when I run "runqemu genericx86-64 ovmf", grub failed
to boot, it's a known issue has been fixed in grub upstream, backport
the fix.

(From OE-Core rev: 6992437d725f9cc88da4261814b69aaadc5ef0f2)

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 51eab4bb0cae46c9c32d28986eb97badf47594b7)
Signed-off-by: Chaitanya Vadrevu <chaitanya.vadrevu@ni.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>