net: fec: Fix FEC_ECR_EN1588 being cleared on link-down

FEC_ECR_EN1588 bit gets cleared after MAC reset in `fec_stop()`, which
makes all 1588 functionality shut down, and all the extended registers
disappear, on link-down, making the adapter fall back to compatibility
"dumb mode". However, some functionality needs to be retained (e.g. PPS)
even without link.

Fixes: 6605b730 ("FEC: Add time stamping code and a PTP hardware clock")
Cc: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/netdev/5fa9fadc-a89d-467a-aae9-c65469ff5fe1@lunn.ch/


Signed-off-by: Csókás, Bence <csokas.bence@prolan.hu>
Reviewed-by: Wei Fang <wei.fang@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>