Prevent current step-down going negative

Addressing #11348

Marlin/src/feature/tmc_util.cpp

     #if CURRENT_STEP_DOWN > 0
       // Decrease current if is_otpw is true and driver is enabled and there's been more than 4 warnings
       if (data.is_otpw && st.isEnabled() && st.otpw_count > 4) {
-        st.rms_current(st.getMilliamps() - (CURRENT_STEP_DOWN));
+        st.rms_current(MAX(int16_t(st.getMilliamps() - (CURRENT_STEP_DOWN)), 0));
         #if ENABLED(REPORT_CURRENT_CHANGE)
           st.printLabel();
           SERIAL_ECHOLNPAIR(" current decreased to ", st.getMilliamps());