net: broadcom: bcm4908_enet: reset DMA rings sw indexes properly

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: 4feffead

 ("net: broadcom: bcm4908enet: add BCM4908 controller driver")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>