net: enetc: use generic interfaces to get phc_index for ENETC v1

net: enetc: use generic interfaces to get phc_index for ENETC v1

The commit 61f132ca8c46 ("ptp: add helpers to get the phc_index by
of_node or dev") has added two generic interfaces to get the phc_index
of the PTP clock. This eliminates the need for PTP device drivers to
provide custom APIs for consumers to retrieve the phc_index. This has
already been implemented for ENETC v4 and is also applicable to ENETC
v1. Therefore, the global variable enetc_phc_index is removed from the
driver. ENETC v1 now uses the same interface as v4 to get phc_index.

Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20250919084509.1846513-3-wei.fang@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>