staging: comedi: ni_mio_common: fix the (*insn_write) for the freq_out counter

The (*insn_write) functions are supposed to write insn->n data values and
return the number of values written. For this subdevice it only makes sense
to write the last data value.

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>