staging: comedi: addi_apci_3120: fix timer (*insn_read)

The timer subdevice in this driver does not follow the comedi API.
Fix the (*insn_read) to work correctly.

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>