net: fec: delete fec_ptp_disable_hwts()

Commit 34074639

 ("net: fec: fix hardware time stamping by external
devices") was overly cautious with calling fec_ptp_disable_hwts() when
cmd == SIOCSHWTSTAMP and use_fec_hwts == false, because use_fec_hwts is
based on a runtime invariant (phy_has_hwtstamp()). Thus, if use_fec_hwts
is false, then fep->hwts_tx_en and fep->hwts_rx_en cannot be changed at
runtime; their values depend on the initial memory allocation, which
already sets them to zeroes.

If the core will ever gain support for switching timestamping layers,
it will arrange for a more organized calling convention and disable
timestamping in the previous layer as a first step. This means that the
code in the FEC driver is not necessary in any case.

The purpose of this change is to arrange the phy_has_hwtstamp() code in
a way in which it can be refactored away into generic logic.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Wei Fang <wei.fang@nxp.com>
Link: https://lore.kernel.org/r/20230801142824.1772134-8-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>