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/cf_spi.c b/drivers/spi/cf_spi.c
index a883da9..afe7917 100644
--- a/drivers/spi/cf_spi.c
+++ b/drivers/spi/cf_spi.c
@@ -330,12 +330,10 @@
 	if (!spi_cs_is_valid(bus, cs))
 		return NULL;
 
-	cfslave = malloc(sizeof(struct cf_spi_slave));
+	cfslave = spi_alloc_slave(struct cf_spi_slave, bus, cs);
 	if (!cfslave)
 		return NULL;
 
-	cfslave->slave.bus = bus;
-	cfslave->slave.cs = cs;
 	cfslave->baudrate = max_hz;
 
 	/* specific setup */