staging: comedi: ni_tio: fix ni_tio_insn_read()

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

Fix this function to work line the core expects.

For aesthetics, factor out the code that reads the SM_Save_Reg.

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