Fix a few recent (STM32F1 DOGM, SPI) warnings (#14669)

Marlin/src/HAL/HAL_STM32F1/HAL.cpp

@@ -260,7 +260,11 @@ extern "C" {
 void HAL_adc_init(void) {
   // configure the ADC
   adc.calibrate();
-  adc.setSampleRate(ADC_SMPR_41_5); // ?
+  #if F_CPU > 72000000
+    adc.setSampleRate(ADC_SMPR_71_5); // 71.5 ADC cycles
+  #else
+    adc.setSampleRate(ADC_SMPR_41_5); // 41.5 ADC cycles
+  #endif
   adc.setPins(adc_pins, ADC_PIN_COUNT);
   adc.setDMA(HAL_adc_results, (uint16_t)ADC_PIN_COUNT, (uint32_t)(DMA_MINC_MODE | DMA_CIRC_MODE), nullptr);
   adc.setScanMode();

Marlin/src/HAL/HAL_STM32F1/HAL_spi_STM32F1.cpp

@@ -104,7 +104,7 @@ void spiInit(uint8_t spiRate) {
  * @details
  */
 uint8_t spiRec(void) {
-  uint8_t returnByte = SPI.transfer(0xFF);
+  uint8_t returnByte = SPI.transfer(ff);
   return returnByte;
 }
 

Marlin/src/HAL/HAL_STM32F1/STM32F1_flag_script.py

@@ -4,7 +4,6 @@ import sys
 #dynamic build flags for generic compile options
 if __name__ == "__main__":
   args = " ".join([ "-std=gnu11",
-                    "-std=gnu++11",
                     "-Os",
                     "-mcpu=cortex-m3",
                     "-mthumb",