[nrf528xx] fix synchronous GetRssi functionality (#4537)

This commit aligns otPlatRadioGetRssi implementation with the latest
changes to radio driver.

Without this change, for example, Channel Monitor and Energy Scanning
with duration of 0 (energy scan 0 command) does not work.

examples/platforms/nrf528xx/src/radio.c

@@ -71,6 +71,8 @@
 #define FRAME_PENDING_OFFSET  1            ///< Byte containing pending bit (+1 for frame length byte).
 #define FRAME_PENDING_BIT     (1 << 4)     ///< Frame Pending bit.
 
+#define RSSI_SETTLE_TIME_US   40           ///< RSSI settle time in microseconds.
+
 #if defined(__ICCARM__)
 _Pragma("diag_suppress=Pe167")
 #endif
@@ -413,6 +415,12 @@ int8_t otPlatRadioGetRssi(otInstance *aInstance)
 {
     OT_UNUSED_VARIABLE(aInstance);
 
+    // Ensure the RSSI measurement is done after RSSI settling time.
+    // This is necessary for the Channel Monitor feature which quickly switches between channels.
+    NRFX_DELAY_US(RSSI_SETTLE_TIME_US);
+
+    nrf_802154_rssi_measure_begin();
+
     return nrf_802154_rssi_last_get();
 }