STM32H7: FDCAN: use PLL1Q frequency for calculations

Author: facchinm

targets/TARGET_STM/can_api.c

@@ -132,8 +132,10 @@ static void _can_init_freq_direct(can_t *obj, const can_pinmap_t *pinmap, int hz
     int ntq = HAL_RCCEx_GetPeriphCLKFreq(RCC_PERIPHCLK_FDCAN) / hz;
 #else
     // STM32H7 doesn't support yet HAL_RCCEx_GetPeriphCLKFreq for FDCAN
-    // Internal ST ticket 92465
-    int ntq = 10000000 / hz;
+    // We use PLL1.Q clock right now so get its frequency
+    PLL1_ClocksTypeDef pll1_clocks;
+    HAL_RCCEx_GetPLL1ClockFreq(&pll1_clocks);
+    int ntq = pll1_clocks.PLL1_Q_Frequency / hz;
 #endif
 
     int nominalPrescaler = 1;