vfio: fix FreeBSD clear group stub

[ upstream commit bf8b792 ]

On FreeBSD, `rte_vfio_clear_group()` was returning 0 even though this
function is not valid for FreeBSD, and is called out to return error in
doxygen comments.
Fix the return value to match documentation.

Fixes: c564a2a ("vfio: expose clear group function for internal usages")

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>

lib/librte_eal/freebsd/eal/eal.c

@@ -1028,7 +1028,7 @@ int rte_vfio_noiommu_is_enabled(void)
 
 int rte_vfio_clear_group(__rte_unused int vfio_group_fd)
 {
-	return 0;
+	return -1;
 }
 
 int