plustek_pp: Fix [-Wshift-negative-value] compiler warning

backend/plustek_pp.c

@@ -216,9 +216,9 @@ static const SANE_String_Const halftone_list[] =
 
 static const SANE_Range percentage_range =
 {
-	-100 << SANE_FIXED_SCALE_SHIFT, /* minimum 		*/
+	SANE_FIX(-100),         /* minimum      */
-	 100 << SANE_FIXED_SCALE_SHIFT, /* maximum 		*/
+	SANE_FIX( 100),         /* maximum      */
-	   1 << SANE_FIXED_SCALE_SHIFT  /* quantization */
+	SANE_FIX(   1)          /* quantization */
 };
 
 /*