Cancel reader before closing the SerialPort
Cancel reader before closing the SerialPort

This change will be required once implementations of the close() method
are updated to implicitly call cancel() on the port's ReadableStream as
otherwise the stream will still be locked when close() is called and
trigger an error.

GoogleChromeLabs/serial-terminal,GoogleChromeLabs/serial-terminal,GoogleChromeLabs/serial-terminal,GoogleChromeLabs/telnet-client,GoogleChromeLabs/telnet-client,GoogleChromeLabs/telnet-client