drivers: spi: consider command bytes when sending transfers

Command bytes are part of the written bytes and they should be taken into
account when sending a spi transfer.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>

include/spi.h

@@ -89,7 +89,7 @@ struct dm_spi_slave_platdata {
  * @max_read_size:	If non-zero, the maximum number of bytes which can
  *			be read at once.
  * @max_write_size:	If non-zero, the maximum number of bytes which can
- *			be written at once, excluding command bytes.
+ *			be written at once.
  * @memory_map:		Address of read-only SPI flash access.
  * @flags:		Indication of SPI flags.
  */