rt2800: zero registers of unused TX rings

This is needed if we take over after drivers which use those.

Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Reviewed-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>