fix typo in usart_init
Merge pull request #22 from arcbbb/fix-master

fix typo in usart_init
1. Do irq_save & irq_return only in necessary case.(context switch).
2. Use IRQ_HANDLER to implement board's HW irq.

TODO:
1. It would crash by using O1 optimization ccompile flag.
2. Hw IRQ Declaration would use too much space.
Merge branch 'master' of https://github.com/georgekang/f9-kernel