spi: fix missing offload_flags doc

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: 700a281905f2 ("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>