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/bfin_spi.c b/drivers/spi/bfin_spi.c
index e080bec..ab2e8b9 100644
--- a/drivers/spi/bfin_spi.c
+++ b/drivers/spi/bfin_spi.c
@@ -182,12 +182,10 @@
 		default: return NULL;
 	}
 
-	bss = malloc(sizeof(*bss));
+	bss = spi_alloc_slave(struct bfin_spi_slave, bus, cs);
 	if (!bss)
 		return NULL;
 
-	bss->slave.bus = bus;
-	bss->slave.cs = cs;
 	bss->mmr_base = (void *)mmr_base;
 	bss->ctl = SPE | MSTR | TDBR_CORE;
 	if (mode & SPI_CPHA) bss->ctl |= CPHA;