ipg: plug Tx completion leak

The Tx skb release could not free more than one skb per call.
Add it to the fact that the xmit handler does not check for
a queue full condition and you have a recipe to leak quickly.

Let's release every pending Tx descriptor which has been given
back to the host CPU by the network controller. The xmit handler
suggests that it is done through the IPG_TFC_TFDDONE bit.

Remove the former "curr" computing: it does not produce anything
usable in its current form.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>