sfc: Remove dependency of PTP on having a dedicated channel

We need a dedicated channel on Siena to ensure we can match up
the separate RX and timestamp events for each PTP packet.  We won't
do this for EF10 as timestamps are delivered inline.

Pass a channel index of 0 to MC_CMD_PTP_OP_ENABLE when there is no
dedicated channel.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>