staging: comedi: ni_daq_700: fix return for insn_config function

The comedi insn_config functions should return < 0 for errors or
the number of data elements used to perform the command, this value
is available as 'insn->n'. Return that instead of the open coded
number to better indicate what the return means.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>