ENGR00172303 - FEC : fix 'eth0: tx queue full!' issue.

ENGR00172303 - FEC : fix 'eth0: tx queue full!' issue.

The issue is hard to reproduce in normal envrionment. And
the reproduce rate is about 40% when doing VTE auto test.

while the driver did report being busy when the link is down
or no transmission buffers are available, it did not stop the
queue, causing instant retries. furthermore, transmission being
triggered with link down was caused by unconditional queue
wakes, especially on timeouts.

Now, wake queue only if link is up and transmission buffers
are available, and dont forget to wake queue when link has
been adjusted. next, add stop queue notification upon driver
induced transmission problems, so network stack has a chance
to handle the situation.

Signed-off-by: Fugang Duan  <B38611@freescale.com>