need to ensure kpreempt_disable() for softint_schedule().

fix the locking in ohci_timeout_task(), ohci_device_isoc_start(),
ohci_device_isoc_abort() and ohci_device_isoc_close().

uaudio(4) can still play with these, but mixerctl -a against it will
hang the writer.  however, mixerctl continues to run in a tight loop
and the system isn't soft-locked up at this point, an advance from
how it is in -current.

(mrg)