generic_ehci_fdt: fix driver softc size

This subclass declares its own softc structure adding necessary members
after the embedded ehci_softc_t. The full size of the struct must be
included in the driver declaration, otherwise the allocation backing the
softc is not guaranteed to be large enough.

Reported by: KASAN
Reviewed by: jrtc27, manu
Fixes: 7a58744fd0f1 ("Split out the attachment from the generic-ehci driver")
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57951