Apply missed const& SPI optimization (#17734)

Followup to eebb68cd7f

Marlin/src/HAL/LPC1768/HAL_SPI.cpp

@@ -213,7 +213,7 @@
 
 void SPIClass::begin() { spiBegin(); }
 
-void SPIClass::beginTransaction(SPISettings cfg) {
+void SPIClass::beginTransaction(const SPISettings &cfg) {
   uint8_t spiRate;
   switch (cfg.spiRate()) {
     case 8000000: spiRate = 0; break;

Marlin/src/HAL/LPC1768/include/SPI.h

@@ -31,7 +31,7 @@
 class SPISettings {
   public:
     SPISettings(uint32_t speed, int, int) : spi_speed(speed) {};
-    uint32_t spiRate() { return spi_speed; }
+    uint32_t spiRate() const { return spi_speed; }
   private:
     uint32_t spi_speed;
 };