-Fix (#101): some MIDI notes failed to send a note off command

Errors introduced in commit 9f94af2855e26acb94885542347289dcd290ed03:
- MPU_NoteOn: duration decrement was lost.
- MPU_Interrupt: noteOnDuration can be negative, and inequality is wrong.
Merge pull request #186 from wangds/fix101b

-Fix (#101): some MIDI notes failed to send a note off command