drivers/spi: support cmd opcode deduction for spi_crop_chunk()

spi_crop_chunk() currently supports deducting the command length
when determining maximum payload size in a transaction. Add support
for deducting just the opcode part of the command by replacing
deduct_cmd_len field to generic flags field. The two enums supported
drive the logic within spi_crop_chunk():
  SPI_CNTRLR_DEDUCT_CMD_LEN
  SPI_CNTRLR_DEDUCT_OPCODE_LEN

All existing users of deduct_cmd_len were converted to using the
flags field.

BUG=b:65485690

Change-Id: I771fba684f0ed76ffdc8573aa10f775070edc691
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/23491
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Justin TerAvest <teravest@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/southbridge/amd/agesa/hudson/spi.c

@@ -164,7 +164,7 @@ static const struct spi_ctrlr spi_ctrlr = {
 	.xfer = spi_ctrlr_xfer,
 	.xfer_vector = spi_xfer_two_vectors,
 	.max_xfer_size = AMD_SB_SPI_TX_LEN,
-	.deduct_cmd_len = true,
+	.flags = SPI_CNTRLR_DEDUCT_CMD_LEN,
 };
 
 const struct spi_ctrlr_buses spi_ctrlr_bus_map[] = {