ionic: Rework Tx start/stop flow

Currently the driver attempts to wake the Tx queue
for every descriptor processed. However, this is
overkill and can cause thrashing since Tx xmit can be
running concurrently on a different CPU than Tx clean.
Fix this by refactoring Tx cq servicing into its own
function so the Tx wake code can run after processing
all Tx descriptors.

The driver isn't using the expected memory barriers
to make sure the stop/start bits are coherent. Fix
this by  making sure to use the correct memory barriers.

Also, the driver is using the wake API during Tx
xmit even though it's already scheduled. Fix this by
using the start API during Tx xmit.

Signed-off-by: Brett Creeley <brett.creeley@amd.com>
Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>