MIPS: lantiq: Do not enable IRQs in dma open

When a DMA channel is opened the IRQ should not get activated
automatically, this allows it to pull data out manually without the help
of interrupts. This is needed for a workaround in the vrx200 Ethernet
driver.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: David S. Miller <davem@davemloft.net>