* Enable RX and host interrupts during bus probe/attach
* Disable all interrupts (+ host ISR) during bus detach
* Enable TX DONE interrupt only when we start transmitting; clear it when
we're done.
* The RX/TX FIFO depth is still conjecture on my part. I'll fix this
shortly.
* The TX FIFO interrupt isn't an "empty" interrupt, it's an "almost empty"
interrupt. Sigh. So..
* .. in ar933x_bus_transmit(), wait for the FIFO to drain before
continuing.
I dislike having to wait for the FIFO to drain, alas.
Tested:
* Atheros AP121 board, AR9331 SoC.
TODO:
* RX/TX overflow, RX error, BREAK support, etc.
* Figure out the true RX/TX FIFO depth.
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