ssm: Add API to silence most state change debug messages

Otherwise tightly looping SSMs (primarily SPI transfers), will flood the
logs in inappropriate ways.

libfprint/fpi-ssm.h

@@ -96,6 +96,8 @@ GError * fpi_ssm_get_error (FpiSsm *machine);
 GError * fpi_ssm_dup_error (FpiSsm *machine);
 int fpi_ssm_get_cur_state (FpiSsm *machine);
 
+void fpi_ssm_silence_debug (FpiSsm *machine);
+
 /* Callbacks to be used by the driver instead of implementing their own
  * logic.
  */