STM-can-api: Fixed variable name for H7

 - can_frequency uses f instead of hz for can frequency
 - Also added comment to system_clock

Author: m-ecry

targets/TARGET_STM/TARGET_STM32G4/TARGET_STM32G474xx/TARGET_NUCLEO_G474RE/system_clock.c

@@ -105,6 +105,9 @@ uint8_t SetSysClock_PLL_HSE(uint8_t bypass)
     RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
     RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSE;
     RCC_OscInitStruct.PLL.PLLM = RCC_PLLM_DIV6;
+    //! 170MHz as a core frequency for FDCAN is not suitable for many frequencies,
+    //! as it provides low accuracy. When no FDCAN is used, the full capacity of 170 MHz
+    //! should be standard.
 #if DEVICE_CAN
     RCC_OscInitStruct.PLL.PLLN = 80;
 #else

targets/TARGET_STM/can_api.c

@@ -287,7 +287,7 @@ int can_frequency(can_t *obj, int f)
 #else
     // STM32H7 doesn't support yet HAL_RCCEx_GetPeriphCLKFreq for FDCAN
     // Internal ST ticket 92465
-    int ntq = 10000000 / hz;
+    int ntq = 10000000 / f;
 #endif
 
     int nominalPrescaler = 1;