net: fec: fix fec_enet_get_free_txdesc_num

When first initialized, cur_tx points to the 1st
entry in the queue, and dirty_tx points to the last.
At this point, fec_enet_get_free_txdesc_num will
return tx_ring_size -2. If tx_ring_size -2 entries
are now queued, then fec_enet_get_free_txdesc_num
should return 0, but it returns tx_ring_size instead.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: David S. Miller <davem@davemloft.net>