net: mvneta: Don't check for CONFIG_PHYLIB

Drop checking for CONFIG_PHYLIB in mvneta, this is already done in
Kconfig.

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