isdn: divamnt: use y2038-safe ktime_get_ts64() for trace data timestamps

divamnt stores a start_time at module init and uses it to calculate
elapsed time. The elapsed time, stored in secs and usecs, is part of
the trace data the driver maintains for the DIVA Server ISDN cards.
No change to the format of that time data is required.

To avoid overflow on 32-bit systems use ktime_get_ts64() to return
the elapsed monotonic time since system boot.

This is a change from real to monotonic time. Since the driver only
stores elapsed time, monotonic time is sufficient and more robust
against real time clock changes. These new monotonic values can be
more useful for debugging because they can be easily compared to
other monotonic timestamps.

Note elaspsed time values will now start at system boot time rather
than module load time, so they will differ slightly from previously
reported values.

Remove declaration and init of previously unused time constants:
start_sec, start_usec.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>