sf: Use spi_flash_alloc() in each SPI flash driver

Rather than each device having its own way to allocate a SPI flash
structure, use the new allocation function everywhere. This will make it
easier to extend the interface without breaking devices.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mtd/spi/ramtron.c b/drivers/mtd/spi/ramtron.c
index 0999781..5299a6d 100644
--- a/drivers/mtd/spi/ramtron.c
+++ b/drivers/mtd/spi/ramtron.c
@@ -284,15 +284,13 @@
 	return NULL;
 
 found:
-	sn = malloc(sizeof(*sn));
+	sn = spi_flash_alloc(struct ramtron_spi_fram, spi, params->name);
 	if (!sn) {
 		debug("SF: Failed to allocate memory\n");
 		return NULL;
 	}
 
 	sn->params = params;
-	sn->flash.spi = spi;
-	sn->flash.name = params->name;
 
 	sn->flash.write = ramtron_write;
 	sn->flash.read = ramtron_read;