rt2800: disable DMA after firmware load

We can receive frames just after firmware load with current code, so
disable DMA just after firmware is loaded, not before.

Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>