spi: fix missing offload_flags doc

Add offload_flags to the documentation comment for struct spi_transfer.
This was missed when adding the field.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/linux-next/20250212154356.784944ea@canb.auug.org.au/
Fixes: 700a281 ("spi: add offload TX/RX streaming APIs")
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250212-spi-offload-fixes-v1-1-e192c69e3bb3@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: dlech

include/linux/spi/spi.h

@@ -973,6 +973,8 @@ struct spi_res {
  * @rx_sg_mapped: If true, the @rx_sg is mapped for DMA
  * @tx_sg: Scatterlist for transmit, currently not for client use
  * @rx_sg: Scatterlist for receive, currently not for client use
+ * @offload_flags: Flags that are only applicable to specialized SPI offload
+ *	transfers. See %SPI_OFFLOAD_XFER_* in spi-offload.h.
  * @ptp_sts_word_pre: The word (subject to bits_per_word semantics) offset
  *	within @tx_buf for which the SPI device is requesting that the time
  *	snapshot for this transfer begins. Upon completing the SPI transfer,