eth: mcux: Do not set carrier ON if interface is not known

It is possible that the network interface is not yet initialized
when status of the PHY changes. In this case we must not call
net_eth_carrier_on() as that will cause a crash.
This was noticed with mimxrt1050_evk board.

Fixes: #21257

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>