refactor mcp4728 component (#5609)

Co-authored-by: Samuel Sieb <samuel@sieb.net>

esphome/components/mcp4728/output/mcp4728_output.cpp

@@ -0,0 +1,17 @@
+#include "mcp4728_output.h"
+
+#include "esphome/core/helpers.h"
+#include "esphome/core/log.h"
+
+namespace esphome {
+namespace mcp4728 {
+
+void MCP4728Channel::write_state(float state) {
+  const uint16_t max_duty = 4095;
+  const float duty_rounded = roundf(state * max_duty);
+  auto duty = static_cast<uint16_t>(duty_rounded);
+  this->parent_->set_channel_value_(this->channel_, duty);
+}
+
+}  // namespace mcp4728
+}  // namespace esphome