[sub-mac] fix incorrect timer configuration for energy scanning (#4538)

This fixes the implementation of Energy Scanning in case the platform
does not support it, but uses timer with microsecond resolution. This
is not the case with nRF528xx but found it during code inspection.

I've verified that after removing OT_RADIO_CAPS_ENERGY_SCAN
capability, the Energy Scan now works correctly.

src/core/mac/sub_mac.cpp

@@ -431,6 +431,8 @@ exit:
 
 void SubMac::SampleRssi(void)
 {
+    assert(!RadioSupportsEnergyScan());
+
     int8_t rssi = GetRssi();
 
     if (rssi != kInvalidRssiValue)
@@ -443,7 +445,11 @@ void SubMac::SampleRssi(void)
 
     if (TimerMilli::GetNow() < mEnergyScanEndTime)
     {
+#if OPENTHREAD_CONFIG_PLATFORM_USEC_TIMER_ENABLE
+        mTimer.StartAt(mTimer.GetFireTime(), kEnergyScanRssiSampleInterval * 1000UL);
+#else
         mTimer.StartAt(mTimer.GetFireTime(), kEnergyScanRssiSampleInterval);
+#endif
     }
     else
     {