[simple-scan] Allow Brother DS-720D to switch back to single side.

[Robert Ancell <rancell src gnome org>]

commit 8daf8d18e08be2c80237c35cad9ac67eef9b469b
Author: Curt Meyers <boilerbots gmail com>
Date:   Sun Mar 29 17:13:50 2020 -0700

    Allow Brother DS-720D to switch back to single side.
    
    Patch from Bartosz, tested on my Brother DS-720D. Allows
    switch back from Duplex to Single. This is a mobile scanner
    so it always requires ADF.

 src/scanner.vala | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/src/scanner.vala b/src/scanner.vala
index 75474af..40be91d 100644
--- a/src/scanner.vala
+++ b/src/scanner.vala
@@ -948,7 +948,8 @@ public class Scanner : Object
                 string[] adf_front_sources =
                 {
                     "ADF Front",
-                    Sane.I18N ("ADF Front")
+                    Sane.I18N ("ADF Front"),
+                    "ADF"
                 };
 
                 string[] adf_back_sources =