sfc: Only cancel the PPS workqueue if it exists

The workqueue only exists for the primary PF. For other functions
we hit a WARN_ON in kernel/workqueue.c.

Fixes: 7c236c43 ("sfc: Add support for IEEE-1588 PTP")
Signed-off-by: Martin Habets <mhabets@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>