MAINTAINERS: linux-fbdev is moderated

While linux-fbdev is subscribers-only, non-subscribers are not plainly
rejected, but moderated, so the casual patch/comment/question comes through.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>