spi-generic: Print an error when trying to use a non-existent bus

...because I just spent hours chasing a refactoring bug that would have
been way more obvious with a little more error transparency in here.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: I3354ff0370ae79f05e5c37d292ac16d446898606
Reviewed-on: https://review.coreboot.org/c/coreboot/+/57573
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/drivers/spi/spi-generic.c

@@ -123,8 +123,10 @@ int spi_setup_slave(unsigned int bus, unsigned int cs, struct spi_slave *slave)
 		}
 	}
 
-	if (slave->ctrlr == NULL)
+	if (slave->ctrlr == NULL) {
+		printk(BIOS_ERR, "Can't find SPI bus %u\n", bus);
 		return -1;
+	}
 
 	slave->bus = bus;
 	slave->cs = cs;