sfc: Disable PTP on EF10 until we're ready to handle inline RX timestamps

Unlike Siena where timestamping is provided by a peripheral, EF10
delivers RX timestamps in the packet prefix.  However the driver
doesn't yet support this.

We are also creating a PHC device for each EF10 function, even though
the clock is really shared between all of them.

Disable hardware PTP/timestamping support until it's complete.

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