spi: Clarify transfer_one() w.r.t. spi_finalize_current_transfer()

 * @prepare_message: set up the controller to transfer a single message,

 *                   for example doing DMA mapping.  Called from threaded

 *                   context.

 * @transfer_one: transfer a single spi_transfer. When the

 *	          driver is finished with this transfer it must call

 *	          spi_finalize_current_transfer() so the subsystem can issue

 *                the next transfer

 * @transfer_one: transfer a single spi_transfer.

 *                  - return 0 if the transfer is finished,

 *                  - return 1 if the transfer is still in progress. When

 *                    the driver is finished with this transfer it must

 *                    call spi_finalize_current_transfer() so the subsystem

 *                    can issue the next transfer

 * @unprepare_message: undo any work done by prepare_message().

 * @cs_gpios: Array of GPIOs to use as chip select lines; one per CS

 *	number. Any individual value may be -ENOENT for CS lines that