mtd: spi-nor-core: Check return value of write_disable() in spi_nor_erase()

The cleanup code of spi_nor_erase() function calls write_disable(), but
does not return it's return value even in case of failure. Fix this.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>

drivers/mtd/spi/spi-nor-core.c

@@ -953,7 +953,9 @@ erase_err:
 	if (!ret)
 		ret = err;
 #endif
-	write_disable(nor);
+	err = write_disable(nor);
+	if (!ret)
+		ret = err;
 
 	return ret;
 }