rt2800usb: do not wipe out USB_DMA_CFG settings

We should not reset USB_DMA_CFG on rt2800usb_init_registers() as this
function is called indirectly from rt2800_enable_radio(). If we
do so, we wipe out USB_DMA_CFG settings from rt2800usb_enable_radio().

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>