HardwareSerial: avoid waiting loop

[ABOSTM]

@matthijskooijman ,
about your comment:
552a133#r55763637
Yes, I think you are right. Thanks for pointing out.
I am quite busy now, and I will need more time to dig this deeper.
So I submit this issue to track this point, and don't forget it.

[matthijskooijman]

Sounds good, thanks :-)