spi: Use spi_alloc_slave() in each SPI driver

Rather than each driver having its own way to allocate a SPI slave,
use the new allocation function everywhere. This will make it easier
to extend the interface without breaking drivers.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/spi/tegra_slink.c b/drivers/spi/tegra_slink.c
index 2c41fab..9da5877 100644
--- a/drivers/spi/tegra_slink.c
+++ b/drivers/spi/tegra_slink.c
@@ -81,13 +81,11 @@
 		return NULL;
 	}
 
-	spi = malloc(sizeof(struct tegra_spi_slave));
+	spi = spi_alloc_slave(struct tegra_spi_slave, bus, cs);
 	if (!spi) {
 		printf("SPI error: malloc of SPI structure failed\n");
 		return NULL;
 	}
-	spi->slave.bus = bus;
-	spi->slave.cs = cs;
 	spi->ctrl = &spi_ctrls[bus];
 	if (!spi->ctrl) {
 		printf("SPI error: could not find controller for bus %d\n",