enetc: convert to schedule_work()

Convert system_wq queue_work() to schedule_work() which is
a wrapper around it, since the former is a rare construct.

Fixes: 7294380c

 ("enetc: support PTP Sync packet one-step timestamping")
Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>