spi: Use error return value in sf_ops

spi: Use error return value in sf_ops

Adjust spi_flash_probe_slave() to return an error value instead of a
pointer so we get the correct error return.

Have the caller allocate memory for spi_flash to simplify error handling,
and also so that driver model can use its existing allocated memory.

Add a spi.h include in the sf_params file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>