HAL_ChibiOS: use a 16 bit sysinterval_t on 16 bit timers

prevent mixed size subtraction errors

libraries/AP_HAL_ChibiOS/system.cpp

@@ -33,6 +33,7 @@ static_assert(sizeof(systime_t) == 2, "expected 16 bit systime_t");
 #elif CH_CFG_ST_RESOLUTION == 32
 static_assert(sizeof(systime_t) == 4, "expected 32 bit systime_t");
 #endif
+static_assert(sizeof(systime_t) == sizeof(sysinterval_t), "expected systime_t same size as sysinterval_t");
 
 #if defined(HAL_EXPECTED_SYSCLOCK)
 #ifdef STM32_SYS_CK