staging: mt7621-eth: Fix compilation error

Fix the following comilation error by making sure that "phy_ring_head" is
defined as "dma_addr_t" and change "phy_ring_tail" to match it as both
should be "dma_addr_t".

error: passing argument 3 of ‘dma_alloc_coherent’ from incompatible pointer type
Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/mt7621-eth/mtk_eth_soc.c

@@ -768,9 +768,8 @@ static int mtk_pdma_tx_map(struct sk_buff *skb, struct net_device *dev,
 /* the qdma core needs scratch memory to be setup */
 static int mtk_init_fq_dma(struct mtk_eth *eth)
 {
-	unsigned int phy_ring_head, phy_ring_tail;
+	dma_addr_t dma_addr, phy_ring_head, phy_ring_tail;
 	int cnt = eth->soc->dma_ring_size;
-	dma_addr_t dma_addr;
 	int i;
 
 	eth->scratch_ring = dma_alloc_coherent(eth->dev,