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/kirkwood_spi.c b/drivers/spi/kirkwood_spi.c
index de81064..caa91e3 100644
--- a/drivers/spi/kirkwood_spi.c
+++ b/drivers/spi/kirkwood_spi.c
@@ -49,13 +49,10 @@
 	if (!spi_cs_is_valid(bus, cs))
 		return NULL;
 
-	slave = malloc(sizeof(struct spi_slave));
+	slave = spi_alloc_slave_base(bus, cs);
 	if (!slave)
 		return NULL;
 
-	slave->bus = bus;
-	slave->cs = cs;
-
 	writel(~KWSPI_CSN_ACT | KWSPI_SMEMRDY, &spireg->ctrl);
 
 	/* calculate spi clock prescaller using max_hz */