The SysBus qdev initfn merely calls SysBusDeviceClass::init, so we can
replace it with a realizefn already. This avoids getting into any initfn
ambiguity with the upcoming Faraday EHCI implementation.

Rename internal usb_ehci_initfn() to usb_ehci_realize() to allow to
return Errors from common initialization code as well.
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>