[MIPS] TX49: Fix use of CDEX build_store_reg()

The commit a923660d786a53e78834b19062f7af2535f7f8ad accidently
prevents TX49 from using CDEX.  Use build_dst_pref() only if prefetch
for store was really available.
Signed-off-by: NAtsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/mm/pg-r4k.c

@@ -243,11 +243,10 @@ static void __init __build_store_reg(int reg)
 
 static inline void build_store_reg(int reg)
 {
-	if (cpu_has_prefetch)
-		if (reg)
-			build_dst_pref(pref_offset_copy);
-		else
-			build_dst_pref(pref_offset_clear);
+	int pref_off = cpu_has_prefetch ?
+		(reg ? pref_offset_copy : pref_offset_clear) : 0;
+	if (pref_off)
+		build_dst_pref(pref_off);
 	else if (cpu_has_cache_cdex_s)
 		build_cdex_s();
 	else if (cpu_has_cache_cdex_p)