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

backend/mustek.c

@@ -188,9 +188,9 @@ static SANE_String_Const halftone_list[] = {
 
 /* Range used for brightness and contrast */
 static const SANE_Range percentage_range = {
-  -100 << SANE_FIXED_SCALE_SHIFT,	/* minimum */
-  100 << SANE_FIXED_SCALE_SHIFT,	/* maximum */
-  1 << SANE_FIXED_SCALE_SHIFT	/* quantization */
+  SANE_FIX(-100),	/* minimum */
+  SANE_FIX(100),	/* maximum */
+  SANE_FIX(1)           /* quantization */
 };
 
 /* SCSI command buffers used by the backend */