net: fec: Refactor: rename `adapter` to `fep`

Rename local `struct fec_enet_private *adapter` to `fep` in `fec_ptp_gettime()` to match the rest of the driver

Signed-off-by: Csókás Bence <csokas.bence@prolan.hu>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>