use dev->flag as control arg

in rt_serial_open(),an int is used to get the serial flag for control arguments,
so ,it's possible to use dev->flag as arguments directly ,rather than use an int
 to check and copy a device flag.
seperate open ops from rt_serial_init() ,add close ops in rt_serial_close()

rt_serial_init() function just init the serial ,but do not open it .After rt_serial_open() is called ,the serial should be opened(if some interrupt modes feature should be also opened ,by calling serial->ops->control).
rt_serial_close() should close the serial(interrupt modes feature should also be closed according to device flag passed by control arguments).