cris: Consider the TLB valid bit on writes to the TLB

When updating the guest TLB we only need to flush previous
mappings from the entry written if the entry was valid.

Also fixes a compiler warning reported by Blue Swirl.
Signed-off-by: NEdgar E. Iglesias <edgar.iglesias@gmail.com>

target-cris/op_helper.c

@@ -164,7 +164,9 @@ void helper_movl_sreg_reg (uint32_t sreg, uint32_t reg)
 
 			D_LOG("tlb flush vaddr=%x v=%d pc=%x\n", 
 				  vaddr, tlb_v, env->pc);
-			tlb_flush_page(env, vaddr);
+			if (tlb_v) {
+				tlb_flush_page(env, vaddr);
+			}
 		}
 	}
 #endif