bgmac: add support for Byte Queue Limits

This makes it possible to use some more advanced queuing
techniques with this driver.

When multi queue support will be added some changes to Byte Queue
handling is needed.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>