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The operation state machine is now polled both when the HAL device's specified
interrupt fires, and also on TickerV. TickerV is the minimum requirement to
enable background transfers, since some state changes (not least timeouts)
won't generate a device interrupt, so would otherwise never complete.

Some existing implementations of HAL devices don't specify the interrupt
number correctly. If we were to enable interrupts in the SD controller for
these, we wouldn't be acking the correct interrupt in the interrupt
controller. The kernel's default interrupt handler traditionally disabled
unhandled interrupt sources, but this no longer seems to work and we end up in
an infinite loop as kernel interrupt dispatch unwinds with the interrupt still
pending. Even if this were fixed, failing to ack a genuine interrupt would
still be undesirable behaviour in the case of shared interrupt lines, because
then the other interrupts sharing the line would be collate...