cxgb3: Fix sparse warning and micro-optimize is_pure_response()

The function is_pure_response() does "ntohl(var) & const" and then
essentially just tests whether the result is 0 or not; this can be done
more efficiently by computing "var & htonl(const)" instead and doing the
byte swap at compile time instead of run time.

This change slightly shrinks the compiled code; eg on x86-64 we save a
couple of bswapl instructions:

add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-8 (-8)
function                                     old     new   delta
t3_sge_intr_msix_napi                        544     536      -8

and this also has the pleasant side effect of fixing a sparse warning:

    drivers/net/cxgb3/sge.c:2313:15: warning: restricted degrades to integer
Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/cxgb3/sge.c

@@ -2306,7 +2306,7 @@ next_fl:
 
 static inline int is_pure_response(const struct rsp_desc *r)
 {
-	u32 n = ntohl(r->flags) & (F_RSPD_ASYNC_NOTIF | F_RSPD_IMM_DATA_VALID);
+	__be32 n = r->flags & htonl(F_RSPD_ASYNC_NOTIF | F_RSPD_IMM_DATA_VALID);
 
 	return (n | r->len_cq) == 0;
 }