[xburst] sound jz4740_pcm: Don't request dma channel mor then once.

It is possible that the hw_params callback is called multiple times when using
the oss emulation layer. Thus we need to check whether the dma channel has
already been requested otherwise we'll end up with all dma channels being
requested by sound driver.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@20369 3c298f89-4303-0410-b956-a3cf2f4a3e73