spi: spi-mem: Add missing description for data.nbytes field

Add a description for spi_mem_op.data.nbytes to the kerneldoc header.

Fixes: c36ff266 ("spi: Extend the core to ease integration of SPI memory controllers")
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

include/linux/spi/spi-mem.h

@@ -79,6 +79,8 @@ enum spi_mem_data_dir {
  * @dummy.buswidth: number of IO lanes used to transmit the dummy bytes
  * @data.buswidth: number of IO lanes used to send/receive the data
  * @data.dir: direction of the transfer
+ * @data.nbytes: number of data bytes to send/receive. Can be zero if the
+ *		 operation does not involve transferring data
  * @data.buf.in: input buffer
  * @data.buf.out: output buffer
  */