[weikai] Rename check_channel_down_ for protected-method naming

Dropping `virtual` from the now-`final` WeikaiChannel turned check_channel_down
into a plain protected method, which clang-tidy requires to end with `_`.

esphome/components/weikai/weikai.cpp

@@ -254,7 +254,7 @@ size_t WeikaiGPIOPin::dump_summary(char *buffer, size_t len) const {
 void WeikaiChannel::setup_channel() {
   ESP_LOGCONFIG(TAG, "  Setting up UART %s:%s", this->parent_->get_name(), this->get_channel_name());
   // we enable transmit and receive on this channel
-  if (this->check_channel_down()) {
+  if (this->check_channel_down_()) {
     ESP_LOGCONFIG(TAG, "  Error channel %s not working", this->get_channel_name());
   }
   this->reset_fifo_();
@@ -374,7 +374,7 @@ size_t WeikaiChannel::rx_in_fifo_() {
   return available;
 }
 
-bool WeikaiChannel::check_channel_down() {
+bool WeikaiChannel::check_channel_down_() {
   // to check if we channel is up we write to the LCR W/R register
   // note that this will put a break on the tx line for few ms
   WeikaiRegister &lcr = this->reg(WKREG_LCR);

esphome/components/weikai/weikai.h

@@ -423,7 +423,7 @@ class WeikaiChannel final : public uart::UARTComponent {
 
   /// @brief check if channel is alive
   /// @return true if OK
-  bool check_channel_down();
+  bool check_channel_down_();
 
 #ifdef TEST_COMPONENT
   /// @ingroup test_