sis900: remove TxIDLE

Before "sis900: fix TX completion" patch, TX completion was done on TxIDLE interrupt.
TX completion also was the only thing done on TxIDLE interrupt.
Since "sis900: fix TX completion", TX completion is done on TxDESC interrupt.
So it is not necessary any more to set and to check for TxIDLE.

Eliminate TxIDLE from sis900.
Correct some typos, too.

Signed-off-by: Sergej Benilov <sergej.benilov@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>