sfc: enable cascaded multicast filters in MCFW

After creating event queue 0, check to see if the workaround is enabled,
 and enable it if necessary.  This will be called during PCI probe and
 also when coming back up after a reset.  The nic_data->workaround_26807
 will be used in the future to control the filter insertion behaviour
 based on this workaround.

Only the primary PF can enable this workaround, so tolerate an EPERM
 error and continue.  Otherwise, if any step in the checking and enabling
 of the workaround fails, the event queue must be removed.

We check that workaround is implemented before trying to enable it,
 and store the current workaround setting before trying to change it.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>