ppc: Fix roll over bug in flush_cache()

If we call flush_cache(0xfffff000, 0x1000) it would never
terminate the loop since end = 0xffffffff and we'd roll over
our counter from 0xfffffe0 to 0 (assuming a 32-byte cache line)
Signed-off-by: NKumar Gala <galak@kernel.crashing.org>

lib_ppc/cache.c

@@ -33,14 +33,16 @@ void flush_cache(ulong start_addr, ulong size)
 	start = start_addr & ~(CONFIG_SYS_CACHELINE_SIZE - 1);
 	end = start_addr + size - 1;
 
-	for (addr = start; addr <= end; addr += CONFIG_SYS_CACHELINE_SIZE) {
+	for (addr = start; (addr <= end) && (addr >= start);
+			addr += CONFIG_SYS_CACHELINE_SIZE) {
 		asm volatile("dcbst 0,%0" : : "r" (addr) : "memory");
 		WATCHDOG_RESET();
 	}
 	/* wait for all dcbst to complete on bus */
 	asm volatile("sync" : : : "memory");
 
-	for (addr = start; addr <= end; addr += CONFIG_SYS_CACHELINE_SIZE) {
+	for (addr = start; (addr <= end) && (addr >= start);
+			addr += CONFIG_SYS_CACHELINE_SIZE) {
 		asm volatile("icbi 0,%0" : : "r" (addr) : "memory");
 		WATCHDOG_RESET();
 	}