netdev: octeon_mgmt: Free TX skbufs in a timely manner.

We also reduce the high water mark to 1 so skbufs are not stranded for
long periods of time.  Since we are cleaning after each packet, no
need to do it in the transmit path.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>