arge: attempt to close a transmit race by only enabling the descriptor at the end of setup.

This driver and the linux ag71xx driver both treat the transmit ring
as a circular linked list of descriptors.  There's no "end" pointer
that is ever NULL - instead, it expects the MAC to hit a finished
descriptor (ARGE_DESC_EMPTY) and stop.

Now, since it's a circular buffer, we may end up with the hardware
hitting the beginning of our multi-descriptor frame before we've finished
setting it up. It then DMA's it in, starts sending it, and we finish
writing out the new descriptor.  The hardware may then write its
completion for the next descriptor out; then we do, and when we next
read it it'll show up as "not done" and transmit completion stops.

This unfortunately manifests itself as the transmit queue always
being active and a massive TX interrupt storm.  We need to actively
ACK packets back from the transmit engine and if we don't (eg because
we think the transmit isn't finished but it is) then the unit will
just keep generating interrupts.

I hit this finally with the below testing setup.  This fixed it for me.

Strictly speaking I should put in a sync in between writing out all of
the descriptors and writing out that final descriptor.

Tested:

* QCA9558 SoC (AP135 reference board) w/ arge1 + vlans acting as a
  router, and iperf -d (tcp, bidirectional traffic.)

Obtained from:	Linux OpenWRT (ag71xx_main.c.)

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