fs_enet: Adjust BDs after tx error

This patch fixes an occasional transmit lockup in the mac-fcc which
occurs after a tx error.  The test scenario had the local port set
to autoneg and the other end fixed at 100FD, resulting in a large
number of late collisions.

According to the MPC8280RM 30.10.1.3 (also 8272RM 29.10.1.3), after
a tx error occurs, TBPTR may sometimes point beyond BDs still marked
as ready.  This patch walks back through the BDs and points TBPTR to
the earliest one marked as ready.

Tested on a custom board with a MPC8280.

Signed-off-by: Mark Ware <mware@elphinstone.net>
Signed-off-by: David S. Miller <davem@davemloft.net>