bhyve: Simplify control flow in the xhci device model

We only need to call pci_xhci_xfer_complete() when handling a transfer
to the control endpoint, so move that code into the epid == 1 block and
eliminate a goto. Also remove an unneeded reinitialization of
setup_trb.

No functional change intended.

MFC after: 1 week
Reviewed by: corvink, jhb
Differential Revision: https://reviews.freebsd.org/D37287