8139cp: enable bql

This adds support for byte queue limits on RTL8139C+

Tested on real hardware.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Acked-By: Dave Täht <dave.taht@bufferbloat.net>
Signed-off-by: David S. Miller <davem@davemloft.net>