cosa: use msecs_to_jiffies for conversions

API compliance scanning with coccinelle flagged:
./drivers/net/wan/cosa.c:520:2-18: WARNING:
	timeout (30) seems HZ dependent

Numeric constants passed to schedule_timeout() make the effective
timeout HZ dependent which makes little sense in a device probe.
Fixed up by converting the constant to jiffies with msecs_to_jiffies()

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