bcm4908: fix Ethernet broken state after interface restart

This fixes traffic stalls after ifdown & ifup.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit f8d5bd20b3)

target/linux/bcm4908/patches-5.4/170-net-broadcom-bcm4908_enet-reset-DMA-rings-sw-indexes.patch

@@ -0,0 +1,43 @@
+From: =?UTF-8?q?Rafa=C5=82=20Mi=C5=82ecki?= <rafal@milecki.pl>
+Date: Tue, 22 Jun 2021 07:05:04 +0200
+Subject: [PATCH] net: broadcom: bcm4908_enet: reset DMA rings sw indexes
+ properly
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+Resetting software indexes in bcm4908_dma_alloc_buf_descs() is not
+enough as it's called during device probe only. Driver resets DMA on
+every .ndo_open callback and it's required to reset indexes then.
+
+This fixes inconsistent rings state and stalled traffic after interface
+down & up sequence.
+
+Fixes: 4feffeadbcb2 ("net: broadcom: bcm4908enet: add BCM4908 controller driver")
+Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
+---
+ drivers/net/ethernet/broadcom/bcm4908_enet.c | 6 +++---
+ 1 file changed, 3 insertions(+), 3 deletions(-)
+
+--- a/drivers/net/ethernet/broadcom/bcm4908_enet.c
++++ b/drivers/net/ethernet/broadcom/bcm4908_enet.c
+@@ -174,9 +174,6 @@ static int bcm4908_dma_alloc_buf_descs(s
+ 	if (!ring->slots)
+ 		goto err_free_buf_descs;
+ 
+-	ring->read_idx = 0;
+-	ring->write_idx = 0;
+-
+ 	return 0;
+ 
+ err_free_buf_descs:
+@@ -303,6 +300,9 @@ static void bcm4908_enet_dma_ring_init(s
+ 
+ 	enet_write(enet, ring->st_ram_block + ENET_DMA_CH_STATE_RAM_BASE_DESC_PTR,
+ 		   (uint32_t)ring->dma_addr);
++
++	ring->read_idx = 0;
++	ring->write_idx = 0;
+ }
+ 
+ static void bcm4908_enet_dma_uninit(struct bcm4908_enet *enet)