SPI: Split writes using spi_crop_chunk()

SPI controllers in Intel and AMD bridges have a slightly different
restriction on how long transactions they can handle.

Change-Id: I3d149d4b7e7e9633482a153d5e380a86c553d871
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/6163
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>

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

@@ -42,6 +42,7 @@ static int bus_claimed = 0;
 #define SPI_REG_CNTRL11		0xd
  #define CNTRL11_FIFOPTR_MASK	0x07
 
+#define AMD_SB_SPI_TX_LEN	64
 
 static u32 spibar;
 
@@ -86,6 +87,11 @@ void spi_init(void)
 	spibar = pci_read_config32(dev, 0xA0) & ~0x1F;
 }
 
+unsigned int spi_crop_chunk(unsigned int cmd_len, unsigned int buf_len)
+{
+	return min(AMD_SB_SPI_TX_LEN - cmd_len, buf_len);
+}
+
 int spi_xfer(struct spi_slave *slave, const void *dout,
 		unsigned int bytesout, void *din, unsigned int bytesin)
 {