sfc: store PTP filters in a list

Instead of using a fixed sized array for the PTP filters, use a list.

This is not actually necessary at this point because the filters for
multicast PTP are a fixed number, but this is a preparation for the
following patches adding support for unicast PTP.

To avoid confusion with the new struct type efx_ptp_rxfilter, change the
name of some local variables from rxfilter to spec, given they're of the
type efx_filter_spec.

Reported-by: Yalin Li <yalli@redhat.com>
Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Reviewed-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>