HAL_PX4: fixed bug in instantiation of I2C bus threads

we ended up with a thread per device instead of a thread per bus!

libraries/AP_HAL_PX4/I2CDevice.cpp

@@ -23,6 +23,8 @@ namespace PX4 {
 
 uint8_t PX4::PX4_I2C::instance;
 
+DeviceBus I2CDevice::businfo[I2CDevice::num_buses];
+
 /*
   constructor for I2C wrapper class
  */    

libraries/AP_HAL_PX4/I2CDevice.h

@@ -67,7 +67,7 @@ public:
 
 private:
     static const uint8_t num_buses = 2;
-    DeviceBus businfo[num_buses];
+    static DeviceBus businfo[num_buses];
     
     uint8_t _busnum;
     PX4_I2C _px4dev;