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There is an issue sending 1 byte on the SPI bus using DMA, but it only
occurs when the transmit is done for the first time after initialising
the SPI and DMA peripherals.  All other cases (sending 2 or more bytes,
doing send_recv, doing recv first) work okay.  We sidestep this issue by
using polling (not DMA) for all 1 byte transfers.  This is fine because
a 1 byte transfer can't be interrupted and doesn't need the benefits of
DMA (and using polling for this case is more efficient).

Resolves #1456.