extmod/machine_spi: Simplify SPI xfer function to only take one buf len.

There is no need to take src_len and dest_len arguments.  The case of
reading-only with a single output byte (originally src_len=1, dest_len>1)
is now handled by using the output buffer as the input buffer, and using
memset to fill the output byte into this buffer.  This simplifies the
implementations of the spi_transfer protocol function.