Previous 0 was always returned.

This way all the 64 bytes available can be filled up

This reverts commit bc7115b104428f9bea4fdfa1b6af245085f6f542.

Some serial libraries opens the serial port BEFORE setting the baudrate.
This patch allows finer control on the reset condition, since it's
checked on open/close (DTR) AND on serial port parameter change.

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