Fix Extended Message Filter count in STM CAN API

As per STM32H7-series reference manuals:
"Up to 64 filter elements can be configured for 29-bit extended IDs."
This commit fixes a bug which prevented receiving CAN-messages
with extended IDs.

Author: SamuA-AP

targets/TARGET_STM/can_api.c

@@ -171,7 +171,7 @@ static void _can_init_freq_direct(can_t *obj, const can_pinmap_t *pinmap, int hz
      * for STM32H7 platforms
      */
     obj->CanHandle.Init.StdFiltersNbr = 128; // to be aligned with the handle parameter in can_filter
-    obj->CanHandle.Init.ExtFiltersNbr = 128; // to be aligned with the handle parameter in can_filter
+    obj->CanHandle.Init.ExtFiltersNbr = 64; // to be aligned with the handle parameter in can_filter
 #else
     /* The number of Standard and Extended ID filters are initialized to the maximum possile extent 
      * for STM32G0x1, STM32G4 and STM32L5  platforms