drivers: ethernet: mcux: improve receive timestamp accuracy

When ENET_ENHANCEDBUFFERDESCRIPTOR_MODE enabled,
MAC will automatic capture receive nanosecond from 1588TMR
and return back to ENET_ReadFrame. It is a highest accuracy
recv timestamp_ns, we do not need manually read from 1588TMR.

By this change, receive timestamp accuracy increase
from 20us to 200ns above.

Signed-off-by: Chen Caidy <chen@caidy.cc>