AP_HAL: AP_HAL_Linux: add comment to I2CDevice::transfer()

If I2CDevice::transfer() has to do nothing it returns false. This can be
misleading, as this might feel contradictory.

Let's spend a comment on that.

Signed-off-by: Ralf Ramsauer <ralf.ramsauer@othr.de>

libraries/AP_HAL_Linux/I2CDevice.cpp

@@ -168,6 +168,7 @@ bool I2CDevice::transfer(const uint8_t *send, uint32_t send_len,
         nmsgs++;
     }
 
+    /* interpret it as an input error if nothing has to be done */
     if (!nmsgs) {
         return false;
     }