sf: dataflash: Minor cleanups

- fix single line comments
- remove unneeded spaces
- ascending order of include files
- rename SPI DATAFLASH to dataflash
- rename SPI DataFlash to dataflash
- return NULL replaced with error code

Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: York Sun <york.sun@nxp.com>
Signed-off-by: Jagan Teki <jagan@openedev.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>