MIPS: lantiq: dma: reset correct number of channel

Different SoCs have a different number of channels, e.g .:
* amazon-se has 10 channels,
* danube+ar9 have 20 channels,
* vr9 has 28 channels,
* ar10 has 24 channels.

We can read the ID register and, depending on the reported
number of channels, reset the appropriate number of channels.

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>