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/atmel_spi.c b/drivers/spi/atmel_spi.c
index ce7d460..f4b1bad 100644
--- a/drivers/spi/atmel_spi.c
+++ b/drivers/spi/atmel_spi.c
@@ -84,12 +84,10 @@
 	if (mode & SPI_CPOL)
 		csrx |= ATMEL_SPI_CSRx_CPOL;
 
-	as = malloc(sizeof(struct atmel_spi_slave));
+	as = spi_alloc_slave(struct atmel_spi_slave, bus, cs);
 	if (!as)
 		return NULL;
 
-	as->slave.bus = bus;
-	as->slave.cs = cs;
 	as->regs = regs;
 	as->mr = ATMEL_SPI_MR_MSTR | ATMEL_SPI_MR_MODFDIS
 #if defined(CONFIG_AT91SAM9X5) || defined(CONFIG_AT91SAM9M10G45)