net: mvneta: Disable fixed PHY code if PHY_FIXED is not compiled in

Guard the code handling the fixed PHY case by
CONFIG_IS_ENABLED(PHY_FIXED).

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>