bhyve: usb: marginally improve debugging output

[kevans]

Summary

Note when we pci_xhci_assert_interrupt, and add the trb type to
pci_xhci_cmd_reset_ep. These are both relatively straightforward.

Sponsored by: The FreeBSD Foundation (aokblast)

[bnovkov]

One minor comment, LGTM otherwise.

usr.sbin/bhyve/pci_xhci.c

1208

Maybe it would make sense to add an %s modifier to the DPRINTF macro.