extmod/network_wiznet5k: Rearrange the function wiznet5k_poll().

To have just one exit and a more compact flag test.  This is just a style
change without impact to the functionality.

extmod/network_wiznet5k.c

@@ -325,10 +325,7 @@ STATIC void wiznet5k_lwip_init(wiznet5k_obj_t *self) {
 
 void wiznet5k_poll(void) {
     wiznet5k_obj_t *self = &wiznet5k_obj;
-    if (!(self->netif.flags & NETIF_FLAG_UP) ||
-        !(self->netif.flags & NETIF_FLAG_LINK_UP)) {
-        return;
-    }
+    if ((self->netif.flags & (NETIF_FLAG_UP | NETIF_FLAG_LINK_UP)) == (NETIF_FLAG_UP | NETIF_FLAG_LINK_UP)) {
         uint16_t len;
         while ((len = wiznet5k_recv_ethernet(self)) > 0) {
             if (self->trace_flags & TRACE_ETH_RX) {
@@ -342,6 +339,7 @@ void wiznet5k_poll(void) {
                 }
             }
         }
+    }
     wizchip_clrinterrupt(IK_SOCK_0);
     #if _WIZCHIP_ == W5100S
     setSn_IR(0, Sn_IR_RECV); // W5100S driver bug: must write to the Sn_IR register to reset the IRQ signal