bhyve: Simplify control flow in the xhci device model

[markj]

Summary

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. Remove an unneeded reinitialization of setup_trb. No
functional change intended.

MFC after: 1 week

[jhb]

usr.sbin/bhyve/pci_xhci.c

1856–1857

I understand why it's there, but having back-to-back assignments of err like this is potentially a bit confusing. One option might be using separate variables for USB errors vs XHCI errors (either something verbose like usb_error and xhci_error or maybe something shorter like uerror and xerror)?

[markj]

Use different variables for different error classes.