eth: mcux: Replace DT_ALIAS() macro with DT_NODELABEL() for fixed link

After the commit SHA1: 370d0274 the
alias for 'eth' was removed. As a result DT_ALIAS(eth) for the mcux
enet driver was not providing a valid DTS node reference.

As a result the 'fixed-link' child node property was not recognized at
all.

The 'enet' is a valid DTS label for mcux enet driver, so lets use it
instead.

Signed-off-by: Lukasz Majewski <lukma@denx.de>