[U-Boot] [PATCH RESEND-WITH-JUSTIFICATION] spi: soft_spi: Support NULL din/dout buffers

[Andrew Ruder]

I further justify this with a list of drivers that depend on half-duplex
SPI operation:

drivers/mmc/mmc_spi.c:          spi_xfer(spi, 2 * 8, tok, NULL, 0);
drivers/mtd/spi/eeprom_m95xxx.c:        if (spi_xfer(slave, 8, buf, NULL, SPI_XFER_BEGIN | SPI_XFER_END))
drivers/mtd/spi/sf_ops.c:               ret = spi_xfer(spi, 8, NULL, &status, 0);
drivers/net/e1000_spi.c:        return e1000_spi_xfer(hw, 8*sizeof(op), op, NULL, intr);
drivers/net/enc28j60.c: spi_xfer(enc->slave, 2 * 8, dout, NULL,
drivers/rtc/m41t94.c:   ret = spi_xfer(slave, 64, buf, NULL, SPI_XFER_BEGIN | SPI_XFER_END);

And an old email first highlighting this issue in 2010 with the soft_spi
driver:

http://article.gmane.org/gmane.comp.boot-loaders.u-boot/75839/match=soft_spi