dma-trace.c: lower log level of dropped logs and stop recursion

Dropping logs is bad but it should not be fatal in itself.

This ERROR is one of the last remaining one, see
thesofproject/sof-test#1075 (comment)

Also switch to the "etrace" and stop using the DMA trace when it's
already saturated.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

src/trace/dma-trace.c

@@ -645,15 +645,9 @@ static void dtrace_add_event(const char *e, uint32_t length)
 			uint32_t tmp_dropped_entries =
 				trace_data->dropped_entries;
 			trace_data->dropped_entries = 0;
-			/*
-			 * this trace_error invocation causes recursion,
-			 * so after it we have to recalculate margin and
-			 * overflow
-			 */
-			tr_err(&dt_tr, "dtrace_add_event(): number of dropped logs = %u",
-			       tmp_dropped_entries);
-			margin = dtrace_calc_buf_margin(buffer);
-			overflow = dtrace_calc_buf_overflow(buffer, length);
+			mtrace_printf(LOG_LEVEL_WARNING,
+				      "dtrace_add_event(): number of dropped logs = %u",
+				      tmp_dropped_entries);
 		}
 	}