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/mxs_spi.c b/drivers/spi/mxs_spi.c
index ffa3c1d..aa999f9 100644
--- a/drivers/spi/mxs_spi.c
+++ b/drivers/spi/mxs_spi.c
@@ -77,15 +77,13 @@
 		return NULL;
 	}
 
-	mxs_slave = calloc(sizeof(struct mxs_spi_slave), 1);
+	mxs_slave = spi_alloc_slave(struct mxs_spi_slave, bus, cs);
 	if (!mxs_slave)
 		return NULL;
 
 	if (mxs_dma_init_channel(MXS_DMA_CHANNEL_AHB_APBH_SSP0 + bus))
 		goto err_init;
 
-	mxs_slave->slave.bus = bus;
-	mxs_slave->slave.cs = cs;
 	mxs_slave->max_khz = max_hz / 1000;
 	mxs_slave->mode = mode;
 	mxs_slave->regs = mxs_ssp_regs_by_bus(bus);