Load ehci_hcd before ohci_hcd and uhci_hcd

In my dmesg:

    Jul 01 01:07:50 nixos kernel: Warning! ehci_hcd should always be loaded before uhci_hcd and ohci_hcd, not after

In Gentoo: https://bugs.gentoo.org/show_bug.cgi?id=260139
In LFS: http://www.linuxfromscratch.org/lfs/view/development/chapter08/kernel.html#conf-modprobe

I'm not really certain how to test it without switching to unstable nixpkgs, which means downloading 440 MB, but maybe Hydra will do that?