drivers/spi: Re-factor spi_crop_chunk

spi_crop_chunk is a property of the SPI controller since it depends
upon the maximum transfer size that is supported by the
controller. Also, it is possible to implement this within spi-generic
layer by obtaining following parameters from the controller:

1. max_xfer_size: Maximum transfer size supported by the controller
(Size of 0 indicates invalid size, and unlimited transfer size is
indicated by UINT32_MAX.)

2. deduct_cmd_len: Whether cmd_len needs to be deducted from the
max_xfer_size to determine max data size that can be
transferred. (This is used by the amd boards.)

Change-Id: I81c199413f879c664682088e93bfa3f91c6a46e5
Signed-off-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: https://review.coreboot.org/19386
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Tested-by: coreboot org <coreboot.org@gmail.com>

src/drivers/spi/spi-generic.c

@@ -88,6 +88,25 @@ int spi_xfer(const struct spi_slave *slave, const void *dout, size_t bytesout,
 	return -1;
 }
 
+unsigned int spi_crop_chunk(const struct spi_slave *slave, unsigned int cmd_len,
+			unsigned int buf_len)
+{
+	const struct spi_ctrlr *ctrlr = slave->ctrlr;
+	unsigned int ctrlr_max;
+
+	if (!ctrlr)
+		return 0;
+
+	ctrlr_max = ctrlr->max_xfer_size;
+
+	assert (ctrlr_max != 0);
+
+	if (ctrlr->deduct_cmd_len && (ctrlr_max > cmd_len))
+		ctrlr_max -= cmd_len;
+
+	return min(ctrlr_max, buf_len);
+}
+
 void __attribute__((weak)) spi_init(void)
 {
 	/* Default weak implementation - do nothing. */