ionic: ignore eexist on rx filter add

Don't worry if the rx filter add firmware request fails on
EEXIST, at least we know the filter is there.  Same for
the delete request, at least we know it isn't there.

Fixes: 2a654540

 ("ionic: Add Rx filter and rx_mode ndo support")
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>