forcedeth: add/modify tx done with limit

There are two tx_done routines to handle tx completion processing. Both
these functions now take in a limit value and return the amount of tx
completions. This will be used by a future patch to determine the total
amount of work done.

Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>