sfc: allow more flexible way of adding filters for PTP

In preparation for the support of PTP over IPv6/UDP and Ethernet in next
patches, allow a more flexible way of adding and removing RX filters for
PTP. Right now, only 2 filters are allowed, which are the ones needed
for PTP over IPv4/UDP.

Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>