usb: Factor usb_insert_transfer out of upm_transfer and make private.

Almost every upm_transfer function starts with:

	mutex_enter(&sc->sc_lock);
	err = usb_insert_transfer(xfer);
	mutex_exit(&sc->sc_lock);
	if (err)
		return err;

Some of them have debug messages sprinkled in here too, or assert
that err == USBD_NORMAL_COMPLETION (alternative is USBD_IN_PROGRESS,
only for pipes with up_running or up_serialise, presumably not
applicable for these types of pipes).  Some of them also assert
xfer->ux_status == USBD_NOT_STARTED, which is guaranteed on entry and
preserved by usb_insert_transer.

Exceptions:

- arch/mips/adm5120/dev/ahci.c ahci_device_isoc_transfer just returns
  USBD_NORMAL_COMPLETION, but I'm pretty sure this is and always has
  been broken anyway, so won't make anything worse (if anything, might
  make it better...)

- external/bsd/dwc2/dwc2.c dwc2_device_bulk_transfer and
  dwc2_device_isoc_transfer _also_ issue dwc2_device_start(xfer)
  under the lock.  This is probably a better way to do it, but let's
  do it uniformly across all HCIs at once.

- rump/dev/lib/libugenhc/ugenhc.c rumpusb_device_bulk_transfer
  sometimes returns USBD_IN_PROGRESS _without_ queueing the transfer,
  in the !rump_threads case.  Not really sure how this is supposed to
  work...  If it actually breaks anything, we can figure it out.


(riastradh)