net: gianfar: convert to ndo_hwtstamp_get() and ndo_hwtstamp_set()

net: gianfar: convert to ndo_hwtstamp_get() and ndo_hwtstamp_set()

New timestamping API was introduced in commit 66f7223039c0 ("net: add
NDOs for configuring hardware timestamping") from kernel v6.6. It is
time to convert the gianfar driver to the new API, so that the
ndo_eth_ioctl() path can be removed completely.

Don't propagate the unnecessary "config.flags = 0;" assignment to
gfar_hwtstamp_get(), because dev_get_hwtstamp() provides a zero
initialized struct kernel_hwtstamp_config.

After removing timestamping logic from gfar_ioctl(), the rest is
equivalent to phy_do_ioctl_running(), so provide that directly as our
ndo_eth_ioctl() implementation.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com> # LS1021A
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20250508143659.1944220-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>