lan7xxx: Require phylib

The lan75xx and lan78xx drivers need to drive their phy via the generic
phylib framework. Let's reflect that dependency in Kconfig, so that we
don't get build errors when phylib does not get selected.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>