ucc_geth: Fix full TX queue processing

commit 7583605b ("ucc_geth: Fix empty
TX queue processing") fixed empty TX queue mishandling, but didn't
account another corner case: when TX queue becomes full.

Without this patch the driver will stop transmiting when TX queue
becomes full since 'bd == ugeth->txBd[txQ]' actually checks for
two things: queue empty or full.

Let's better check for NULL skb, which unambiguously signals an empty
queue.

Signed-off-by: Jiajun Wu <b06378@freescale.com>
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Cc: Stable <stable@vger.kernel.org> [2.6.32]
Signed-off-by: David S. Miller <davem@davemloft.net>