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/xilinx_spi.c b/drivers/spi/xilinx_spi.c
index db01cc2..a82b056 100644
--- a/drivers/spi/xilinx_spi.c
+++ b/drivers/spi/xilinx_spi.c
@@ -85,14 +85,12 @@
 		return NULL;
 	}
 
-	xilspi = malloc(sizeof(*xilspi));
+	xilspi = spi_alloc_slave(struct xilinx_spi_slave, bus, cs);
 	if (!xilspi) {
 		printf("XILSPI error: %s: malloc of SPI structure failed\n",
 				__func__);
 		return NULL;
 	}
-	xilspi->slave.bus = bus;
-	xilspi->slave.cs = cs;
 	xilspi->regs = (struct xilinx_spi_reg *)xilinx_spi_base_list[bus];
 	xilspi->freq = max_hz;
 	xilspi->mode = mode;