sfc: remove expired unicast PTP filters

Filters inserted to support unicast PTP mode might become unused after
some time, so we need to remove them to avoid accumulating many of them.

Refresh the expiration time of a filter each time it's used. Then check
periodically if any filter hasn't been used for a long time (30s) and
remove it.

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>