[][update code for dma free which may cause memory leak]

[Description]
Change code for memory free which may cause memory leak
-- memory leak occurs when user use DRAM for DMAD
   -- SRAM ==1, no need free ring->dma
   -- SRAM ==0, DMAD memory should be freed in mtk_rx_clean

[Release-log]
N/A

Change-Id: I599d3e38c66d67c0e9bfd7afc3e8f773258f97db
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6182630
1 file changed