net: fec: fix hardware time stamping by external devices

Fix support for external PTP-aware devices such as DSA or PTP PHY:

Make sure we never time stamp tx packets when hardware time stamping
is disabled.

Check for PTP PHY being in use and then pass ioctls related to time
stamping of Ethernet packets to the PTP PHY rather than handle them
ourselves. In addition, disable our own hardware time stamping in this
case.

Fixes: 6605b730

 ("FEC: Add time stamping code and a PTP hardware clock")
Signed-off-by: Sergey Organov <sorganov@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Acked-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>