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/fsl_espi.c b/drivers/spi/fsl_espi.c
index eb99e90..28609ee 100644
--- a/drivers/spi/fsl_espi.c
+++ b/drivers/spi/fsl_espi.c
@@ -79,12 +79,10 @@
 	if (!spi_cs_is_valid(bus, cs))
 		return NULL;
 
-	fsl = malloc(sizeof(struct fsl_spi_slave));
+	fsl = spi_alloc_slave(struct fsl_spi_slave, bus, cs);
 	if (!fsl)
 		return NULL;
 
-	fsl->slave.bus = bus;
-	fsl->slave.cs = cs;
 	fsl->mode = mode;
 	fsl->max_transfer_length = ESPI_MAX_DATA_TRANSFER_LEN;