virtio-kbd: modified elem index and mutex lock

virtio-kbd: modified elem index and mutex lock

When elem index was higher than 9, Data was written from index 0.
But kernel read queue from index 0 to index 9 always.
If index 9 is key press event and index 0 is key release event, their order is changed.
So key press event is repeated in X platform.
It is modified that elem index must start from 0.

Signed-off-by: Sungmin Ha <sungmin82.ha@samsung.com>