staging: comedi: addi_apci_1564: use comedi_handle_event() for counters

The counter subdevice can generate an interrupt. Currently send_sig() is used
to let the task know when the interrupt occurs. Use the dev->read_subdev and
comedi_handle_events() instead.

Remove the, now unused, 'tsk_current' member from the private data and the
unnecessary include of <linux/sched.h>.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>