pps: Use pps_lookup_dev to reduce ldisc coupling

commit 03a7ffe4e542310838bac70ef85acc17536b6d7c upstream.

Now that N_TTY uses tty->disc_data for its private data,
'subclass' ldiscs cannot use ->disc_data for their own private data.
(This is a regression is v3.8-rc1)

Use pps_lookup_dev to associate the tty with the pps source instead.

This fixes a crashing regression in 3.8-rc1.

Signed-off-by: George Spelvin <linux@horizon.com>
Acked-by: Rodolfo Giometti <giometti@enneenne.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>