hwclock: drop all samples on reset

On some HW it seems it's possible to get an occasional bad reading of
the PHC (with normal delay), or in a worse case the clock can step due
to a HW/driver bug, which triggers reset of the HW clock instance. To
avoid having a bad estimate of the frequency when the next (good) sample
is accumulated, drop also the last sample which triggered the reset.

test/unit/hwclock.c

@@ -65,7 +65,7 @@ test_unit(void)
       if (HCL_NeedsNewSample(clock, &local_ts))
         HCL_AccumulateSample(clock, &hw_ts, &local_ts, 2.0 * jitter);
 
-      TEST_CHECK(j < 20 || clock->valid_coefs);
+      TEST_CHECK(clock->valid_coefs || clock->n_samples < 2);
 
       if (!clock->valid_coefs)
         continue;