MAINTAINERS: wimax@linuxwimax.org is subscribers-only

Mark it so.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>