extmod/network_wiznet5k: Properly enable interrupt signal on w5100s.

According to the datasheet, the IEN bit to enable the interrupt is in the
MR2 register, not the MR register.

This is just cleanup as the interrupt appears to be enabled by default
after resetting the chip.

Tested on W5100S_EVB_PICO.

extmod/network_wiznet5k.c

@@ -221,7 +221,7 @@ static void wiznet5k_init(void) {
         setSn_IMR(0, Sn_IR_RECV);
         #if _WIZCHIP_ == W5100S
         // Enable interrupt pin
-        setMR(MR2_G_IEN);
+        setMR2(getMR2() | MR2_G_IEN);
         #endif
 
         mp_hal_pin_input(wiznet5k_obj.pin_intn);