SerialPort when flush or close, it will wait too long to close the serialPort

Nobody has claimed this yet.

Code like:
` printer
.hardware("init")
.text('敏捷的棕色狐狸跳过懒狗')
.flush(function(){
// nothing
})
.close();
`
or
`printer
.hardware("init")
.text('敏捷的棕色狐狸跳过懒狗')
.close(function(){
// nothing
});
`
The frist print is OK, but after twice or more, the serialPort will close very very slow.

Start at the SerialPort handling for flush and close, then reproduce the issue with the repeated printer sequence shown in the report. Check how repeated flush or close calls affect shutdown timing; done means the serial port closes promptly after multiple print cycles.

Written by the indexing model from the issue text.