i2c: start() should aquire the bus

From the history it looks like aquire was not added explicitely to start. I do not see
why as it's the condition that must be sent when communication is initiated.

write/read/transfer invoke `aquire()`

Author: 0xc0170

drivers/source/I2C.cpp

@@ -137,6 +137,7 @@ int I2C::read(int ack)
 void I2C::start(void)
 {
     lock();
+    aquire();
     i2c_start(&_i2c);
     unlock();
 }