net: fec: better implementation of iMX6 ERR006358 quirk

Using a (delayed) workqueue for ERR006358 is not correct - a work queue
is a single-trigger device.  Once the work queue has been scheduled, it
can't be re-scheduled until it has been run.  This can cause problems -
with an appropriate packet timing, we can end up with packets queued,
but not sent by the hardware, resulting in the transmit timeout firing.

Re-implement this as per the workaround detailed in the ERR006358
documentation - if there are packets waiting to be sent when we service
the transmit ring, and we see that the transmitter is not running,
kick the transmitter to run the pending entries in the ring.

Testing here with a 10Mbit half duplex link sees the resulting iperf
TCP bandwidth increase from between 1 to 2Mbps to between 8 to 9Mbps.

Acked-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>