Fix bug introduced by commit 52a70114

The return value from lseek is not checked correctly, causing that the
environment is not read or written if an offset is set.

Signed-off-by: Stefano Babic <sbabic@denx.de>

src/uboot_env.c

@@ -403,7 +403,9 @@ static int fileread(struct uboot_flash_env *dev, void *data)
 	if (dev->offset)
 		ret = lseek(dev->fd, dev->offset, SEEK_SET);
 
-	if (!ret)
+	if (ret < 0)
+		return ret;
+
 	ret = read(dev->fd, data, dev->envsize);
 
 	return ret;
@@ -599,7 +601,9 @@ static int filewrite(struct uboot_flash_env *dev, void *data)
 	if (dev->offset)
 		ret = lseek(dev->fd, dev->offset, SEEK_SET);
 
-	if (!ret)
+	if (ret < 0)
+		return ret;
+
 	ret = write(dev->fd, data, dev->envsize);
 
 	fileprotect(dev, true);  // no error handling, keep ret from write