Clean the dma flags of multiq ring buffer int the interface stop
process. This patch fixes that the genet is not running while the
interface is re-enabled.

$ ifup eth0 - running after booting
$ ifdown eth0
$ ifup eth0 - not running and occur tx_timeout

The bcmgenet_dma_disable() in bcmgenet_open() do clean ring16 dma flag
only. If the genet has multiq, the dma register is not cleaned. and
bcmgenet_init_dma() is not done correctly. in case
GENET_V2(tx_queues=4), tdma_ctrl has 0x1e after running
bcmgenet_dma_disable().
Signed-off-by: Jaedon Shin <jaedon.shin@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>