staging: comedi: rti800: change return of rti800_ai_insn_read()

The comedi core expects the (*insn_read) functions to return the
number of data values actually read. Change the return from 'i'
to 'insn->n' to make this clearer.

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