Merge pull request ARMmbed#14675 from 0xc0170/fix_i2c_start

i2c: `start()` should aquire the bus

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();
 }