dt-bindings: usb: meson-g12a-usb: Fix FL-adj property value

An empty snps,quirk-frame-length-adjustment won't cause any change
performed by the driver. Moreover the DT schema validation will fail,
since it expects the property being assigned with some value. So set
fix the example by setting a valid FL-adj value in accordance with
Neil Armstrong comment.

Link: https://lore.kernel.org/linux-usb/20201010224121.12672-16-Sergey.Semin@baikalelectronics.ru/Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20201210090944.16283-16-Sergey.Semin@baikalelectronics.ruSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/amlogic,meson-g12a-usb-ctrl.yaml

@@ -229,6 +229,6 @@ examples:
               interrupts = <30>;
               dr_mode = "host";
               snps,dis_u2_susphy_quirk;
-              snps,quirk-frame-length-adjustment;
+              snps,quirk-frame-length-adjustment = <0x20>;
           };
     };