atm: use msecs_to_jiffies for conversions

API compliance scanning with coccinelle flagged:
./drivers/atm/iphase.c:2621:4-20:
        WARNING: timeout (50) seems HZ dependent

Numeric constants passed to schedule_timeout() make the effective
timeout HZ dependent which does not seem intended.
Fixed up by converting the constant to jiffies with msecs_to_jiffies()
As this driver was introduced in the early 2.3 series it is most
likely assuming HZ=100 so the constant 50 is converted to 500ms.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>