Remove qFind usage from QSerialPortPrivate

qFind is deprecated by now. See discussion at
http://www.mail-archive.com/development@qt-project.org/msg01603.html

Change-Id: I18f5be8e803af645df21d47c41d820b8f7fecfa7
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>