bgmac: simplify tx ring index handling

Keep incrementing ring->start and ring->end instead of pointing it to
the actual ring slot entry. This simplifies the calculation of the
number of free slots.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ethernet/broadcom/bgmac.h b/drivers/net/ethernet/broadcom/bgmac.h
index 3ad965f..5a198d5 100644
--- a/drivers/net/ethernet/broadcom/bgmac.h
+++ b/drivers/net/ethernet/broadcom/bgmac.h
@@ -414,10 +414,10 @@
  * empty.
  */
 struct bgmac_dma_ring {
-	u16 num_slots;
-	u16 start;
-	u16 end;
+	u32 start;
+	u32 end;
 
+	u16 num_slots;
 	u16 mmio_base;
 	struct bgmac_dma_desc *cpu_base;
 	dma_addr_t dma_base;