staging: mt7621-gpio: update TODO file

update TODO file accordly to last changes.
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/mt7621-gpio/TODO

-
 - general code review and clean up
-- ensure device-tree requirements are documented
 
 Cc:  NeilBrown <neil@brown.name>