TTY: hvsi, sanitize uses of tty

- use tty, not hp->tty wherever possible
- pass tty down to some functions and go to step one
- do not defer tty_hangup calls -- it is as simple as schedule_work,
  so might be called with hp->lock held
- do not defer tty buffer flips -- since the driver does not use
  low_latency (it cannot actually), the flip is a simple tail move
  plus schedule_work. It will make our life easier in the next patch.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>