8139cp: reset BQL when ring tx ring cleared

This patch cures transmit timeout's with DHCP observed
while running under KVM. When the transmit ring is cleaned out,
the Byte Queue Limit values need to be reset.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>