MAINTAINERS: fix drivers/staging/sm7xx/

This directory was moved to drivers/staging/sm7xxfb/ by commit
925aa660 ("staging: sm7xxfb: sm7xx becomes sm7xxfb").

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Cc: Teddy Wang <teddy.wang@siliconmotion.com.cn>
Acked-by: Javier Muñoz <jmunhoz@igalia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>