Fix USB model defaults for ppc64

Fix USB model defaults for ppc64

The condition was checking for UHCI (and OHCI for ppc64) availability so
that it can specify the proper device instead of legacy usb.  However,
for ppc64, we don't need to check both OHCI and UHCI, but only OHCI as
that is the legacy default.  The condition is so big that it was just a
matter of time when someone will make a mistake there, so let's use more
lines so that it is visible what the condition checks for.

This fixes usage of -device instead of -usb for ppc64 that supports
pci-usb-ohci and does not support piix3-usb-uhci.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1297020

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>