qed: Flush slowpath tasklet on stop

Today, driver has a synchronization point while closing
the device which synchronizes its slowpath interrupt line.
However, that's insufficient as that ISR would schedule the
slowpath-tasklet - so even after ISR is over it's possible the
handling of the interrupt has not completed.

By doing a disable/enable on the taskelt we guarantee that all
HW events that should no longer be genereated from that point
onward in the flow are truly behind us.

Signed-off-by: Tomer Tayar <Tomer.Tayar@cavium.com>
Signed-off-by: Yuval Mintz <Yuval.Mintz@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>