porting/FreeRTOS: fix IRQ priority

FreeRTOS assumes a max irq priority for IRQs that are allowed to do
system calls. This is set to 5 by default, so we use the highest allowed
for the Radio.

nimble/drivers/nrf5x/src/ble_phy.c

@@ -1657,8 +1657,12 @@ ble_phy_init(void)
 
     /* Set isr in vector table and enable interrupt */
 #ifndef RIOT_VERSION
+#ifdef FREERTOS
+    NVIC_SetPriority(RADIO_IRQn, 5);
+#else
     NVIC_SetPriority(RADIO_IRQn, 0);
 #endif
+#endif
 #if MYNEWT
     NVIC_SetVector(RADIO_IRQn, (uint32_t)ble_phy_isr);
 #else