staging: comedi: ni_mio_common: use insn->n in ni_calib_insn_read()

The `insn_read` handler for the calibration subdevice
(`ni_calib_insn_read()`) currently ignores `insn->n` (the number of
samples to read) and assumes a single sample is to be read into
`data[0]`.  Fortunately, the Comedi core ensures that `data[]` has a
length of at least 16, so there is no problem with array bounds.

The usual Comedi convention for `insn_read` handlers is to read the same
channel `insn->n` times into successive elements of `data[]`, so let's
do that.

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

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