HAL_ChibiOS: added HAL_I2C_MAX_CLOCK define

libraries/AP_HAL_ChibiOS/I2CDevice.cpp

@@ -37,6 +37,12 @@ I2CBus I2CDeviceManager::businfo[ARRAY_SIZE_SIMPLE(I2CD)];
 #define HAL_I2C_BUS_BASE 0
 #endif
 
+// default to 100kHz clock for maximum reliability. This can be
+// changed in hwdef.dat
+#ifndef HAL_I2C_MAX_CLOCK
+#define HAL_I2C_MAX_CLOCK 100000
+#endif
+
 // get a handle for DMA sharing DMA channels with other subsystems
 void I2CBus::dma_init(void)
 {
@@ -56,10 +62,14 @@ I2CDeviceManager::I2CDeviceManager(void)
           drop the speed to be the minimum speed requested
          */
         businfo[i].i2ccfg.op_mode = OPMODE_I2C;
-        businfo[i].i2ccfg.clock_speed = 400000;
+        businfo[i].i2ccfg.clock_speed = HAL_I2C_MAX_CLOCK;
+        if (businfo[i].i2ccfg.clock_speed <= 100000) {
+            businfo[i].i2ccfg.duty_cycle = STD_DUTY_CYCLE;
+        } else {
             businfo[i].i2ccfg.duty_cycle = FAST_DUTY_CYCLE_2;
         }
     }
+}
 
 I2CDevice::I2CDevice(uint8_t busnum, uint8_t address, uint32_t bus_clock, bool use_smbus, uint32_t timeout_ms) :
     _retries(2),