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/altera_spi.c b/drivers/spi/altera_spi.c
index 138d6f4..b53607a 100644
--- a/drivers/spi/altera_spi.c
+++ b/drivers/spi/altera_spi.c
@@ -83,12 +83,10 @@
 	if (!spi_cs_is_valid(bus, cs))
 		return NULL;
 
-	altspi = malloc(sizeof(*altspi));
+	altspi = spi_alloc_slave(struct altera_spi_slave, bus, cs);
 	if (!altspi)
 		return NULL;
 
-	altspi->slave.bus = bus;
-	altspi->slave.cs = cs;
 	altspi->base = altera_spi_base_list[bus];
 	debug("%s: bus:%i cs:%i base:%lx\n", __func__,
 		bus, cs, altspi->base);