AP_HAL_ChibiOS/CAN*Iface: fix C++ One Definition Rule violations

Switch to const extern hal reference, then cast const-ness away to
modify it and store the CAN driver reference during driver init.

This does just replace one undefined behavior with another, but the
latter is not warned about and is used in other parts of the code for
the same purpose.

libraries/AP_HAL_ChibiOS/CANFDIface.cpp

@@ -85,7 +85,7 @@
 #error "Unsupported MCU for FDCAN"
 #endif
 
-extern AP_HAL::HAL& hal;
+extern const AP_HAL::HAL& hal;
 
 #define STR(x) #x
 #define XSTR(x) STR(x)
@@ -568,7 +568,7 @@ bool CANIface::init(const uint32_t bitrate, const uint32_t fdbitrate, const Oper
     if (can_ifaces[self_index_] == nullptr) {
         can_ifaces[self_index_] = this;
 #if !defined(HAL_BOOTLOADER_BUILD)
-        hal.can[self_index_] = this;
+        const_cast <AP_HAL::HAL&> (hal).can[self_index_] = this;
 #endif
     }
 

libraries/AP_HAL_ChibiOS/CanIface.cpp

@@ -84,7 +84,7 @@
 #endif
 
 
-extern AP_HAL::HAL& hal;
+extern const AP_HAL::HAL& hal;
 
 using namespace ChibiOS;
 
@@ -846,7 +846,7 @@ bool CANIface::init(const uint32_t bitrate, const CANIface::OperatingMode mode)
     if (can_ifaces[self_index_] == nullptr) {
         can_ifaces[self_index_] = this;
 #if !defined(HAL_BOOTLOADER_BUILD)
-        hal.can[self_index_] = this;
+        const_cast <AP_HAL::HAL&> (hal).can[self_index_] = this;
 #endif
     }