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-generic.c

@@ -146,56 +146,3 @@ int spi_setup_slave(unsigned int bus, unsigned int cs, struct spi_slave *slave)
 
 	return 0;
 }
-
-static int spi_xfer_combine_two_vectors(const struct spi_slave *slave,
-					struct spi_op *v1, struct spi_op *v2)
-{
-	struct spi_op op = {
-		.dout = v1->dout, .bytesout = v1->bytesout,
-		.din = v2->din, .bytesin = v2->bytesin,
-	};
-	int ret;
-
-	/*
-	 * Combine two vectors only if:
-	 * v1 has non-NULL dout and NULL din and
-	 * v2 has non-NULL din and NULL dout and
-	 *
-	 * In all other cases, do not combine the two vectors.
-	 */
-	if ((!v1->dout || v1->din) || (v2->dout || !v2->din))
-		return -1;
-
-	ret = spi_xfer_single_op(slave, &op);
-	v1->status = v2->status = op.status;
-
-	return ret;
-}
-
-/*
- * Helper function to allow chipsets to combine two vectors if possible. This
- * function can only handle upto 2 vectors.
- *
- * Two vectors are combined if first vector has a non-NULL dout and NULL din and
- * second vector has a non-NULL din and NULL dout. Otherwise, each vector is
- * operated upon one at a time.
- *
- * Returns 0 on success and non-zero on failure.
- */
-int spi_xfer_two_vectors(const struct spi_slave *slave,
-			struct spi_op vectors[], size_t count)
-{
-	int ret;
-
-	assert (count <= 2);
-
-	if (count == 2) {
-		ret = spi_xfer_combine_two_vectors(slave, &vectors[0],
-						&vectors[1]);
-
-		if (!ret || (vectors[0].status != SPI_OP_NOT_EXECUTED))
-			return ret;
-	}
-
-	return spi_xfer_vector_default(slave, vectors, count);
-}