Protected use of subdevice runflags with spinlock, since it is modified

Protected use of subdevice runflags with spinlock, since it is modified
in comedi_event().  Replaced most usage of SDF_RUNNING in subdev_flags
with SRF_RUNNING in runflags so I don't have to protect subdev_flags
with a lock too.  Made mite_release_channel() always disarm and
reset mite channel.  Move ack/confirm of counter interrupts/errors
into ni_tio.  Added ni_tio_set_mite_channel(), which acquires
counter's spinlock before modifiying its mite channel.  Added
some missing locking of mite_channel_lock in ni_mio_common.c.