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

backend/pnm.c

@@ -106,9 +106,9 @@ static const SANE_Word resbit_list[] = {
   200, 210, 225, 240, 255, 270, 285, 300
 };
 static const SANE_Range percentage_range = {
-  -100 << SANE_FIXED_SCALE_SHIFT,	/* minimum */
-  100 << SANE_FIXED_SCALE_SHIFT,	/* maximum */
-  0 << SANE_FIXED_SCALE_SHIFT	/* quantization */
+  SANE_FIX(-100),	/* minimum */
+  SANE_FIX(100),	/* maximum */
+  SANE_FIX(0)           /* quantization */
 };
 static const SANE_Range gamma_range = {
   0,				/* minimum */