net: enetc: use common naming scheme for PTGCR and PTGCAPR registers

The Port Time Gating Control Register (PTGCR) and Port Time Gating
Capability Register (PTGCAPR) have definitions in the driver which
aren't in line with the other registers. Rename these.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>