drivers/spi: reduce confusion in the API

Julius brought up confusion about the current spi api in [1]. In order
alleviate the confusion stemming from supporting x86 spi flash
controllers:

- Remove spi_xfer_two_vectors() which was fusing transactions to
  accomodate the limitations of the spi controllers themselves.
- Add spi_flash_vector_helper() for the x86 spi flash controllers to
  utilize in validating driver/controller current assumptions.
- Remove the xfer() callback in the x86 spi flash drivers which
  will trigger an error as these controllers can't support the api.

[1] https://mail.coreboot.org/pipermail/coreboot/2018-April/086561.html

Change-Id: Id88adc6ad5234c29a739d43521c5f344bb7d3217
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/25745
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/drivers/spi/spi_flash.c

@@ -519,3 +519,51 @@ int spi_flash_ctrlr_protect_region(const struct spi_flash *flash,
 
 	return -1;
 }
+
+int spi_flash_vector_helper(const struct spi_slave *slave,
+	struct spi_op vectors[], size_t count,
+	int (*func)(const struct spi_slave *slave, const void *dout,
+		    size_t bytesout, void *din, size_t bytesin))
+{
+	int ret;
+	void *din;
+	size_t bytes_in;
+
+	if (count < 1 || count > 2)
+		return -1;
+
+	/* SPI flash commands always have a command first... */
+	if (!vectors[0].dout || !vectors[0].bytesout)
+		return -1;
+	/* And not read any data during the command. */
+	if (vectors[0].din || vectors[0].bytesin)
+		return -1;
+
+	if (count == 2) {
+		/* If response bytes requested ensure the buffer is valid. */
+		if (vectors[1].bytesin && !vectors[1].din)
+			return -1;
+		/* No sends can accompany a receive. */
+		if (vectors[1].dout || vectors[1].bytesout)
+			return -1;
+		din = vectors[1].din;
+		bytes_in = vectors[1].bytesin;
+	} else {
+		din = NULL;
+		bytes_in = 0;
+	}
+
+	ret = func(slave, vectors[0].dout, vectors[0].bytesout, din, bytes_in);
+
+	if (ret) {
+		vectors[0].status = SPI_OP_FAILURE;
+		if (count == 2)
+			vectors[1].status = SPI_OP_FAILURE;
+	} else {
+		vectors[0].status = SPI_OP_SUCCESS;
+		if (count == 2)
+			vectors[1].status = SPI_OP_SUCCESS;
+	}
+
+	return ret;
+}