net: fec: return IRQ_HANDLED if fec_ptp_check_pps_event handled it

fec_ptp_check_pps_event will return 1 if FEC_T_TF_MASK caused
an interrupt. Don't return IRQ_NONE in this case.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>