rt2800: introduce wpdma_disable function

Introduce wpdma_disable function to simplify the code.

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