net: mediatek: fix missing free of scratch memory

Scratch memory gets allocated in mtk_init_fq_dma() but the corresponding
code to free it is missing inside mtk_dma_free() causing a memory leak.
With this patch applied, we can run ifconfig up/down several thousand
times without any problems.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>