staging: comedi: ni_mio_common: Use insn->n in ni_calib_insn_write()

The `insn_write` handler for the calibration subdevice
(`ni_calib_insn_write()`) currently ignores `insn->n` (the number of
samples to write) and assumes a single sample is to be written, but
`insn->n` could be 0, meaning no samples should be written, in which
case `data[0]` is invalid.

Change `ni_calib_insn_write()` to only write to the calibration device
if `insn->n > 0`.  There isn't much point writing all the values when
`insn->n > 1`, so just write the last one (`data[insn->n - 1]`).

Also follow the usual Comedi convention and return `insn->n` from the
handler to indicate success (although any non-negative return value will
do as far as the Comedi core is concerned).

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>