net: atl1c: add BQL support

This BQL implementation is mostly derived from its related driver, alx.
Tested on AR8131 (rev c0) [1969:1063]. Saturated a 100mbps link with 5
concurrent runs of netperf. Ping latency dropped from 14ms to 3ms.

Signed-off-by: Ron Angeles <ronangeles@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>