soc/amd/common/spi: add and use define for last FIFO position

The existing define for SPI_FIFO_DEPTH looked a bit suspicious, but
turned out to be correct.

Change-Id: I91e65d922673f5c451a336ae013cb75f87a3fc98
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42076
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/amd/common/block/include/amdblocks/spi.h

@@ -55,7 +55,8 @@ enum spi100_speed {
 #define   SPI_RD4DW_EN_HOST		BIT(15)
 
 #define SPI_FIFO			0x80
-#define   SPI_FIFO_DEPTH		(0xc7 - SPI_FIFO)
+#define SPI_FIFO_LAST_BYTE		0xc7
+#define SPI_FIFO_DEPTH			(SPI_FIFO_LAST_BYTE - SPI_FIFO)
 
 struct spi_config {
 	/*