staging: comedi: ni_labpc: fix labpc_calib_insn_read()

The comedi core expects the (*insn_read) operations to read insn->n
values and return the number of values actually read.

Make this function work like the core expects.

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>