staging: comedi: ssv_dnp: correct insn_bits result

[Part of commit f6b316bcd8c421acd6fa5a6e18b4c846ecb9d965 upstream.
Split from original patch subject: "staging: comedi: ssv_dnp: use
comedi_dio_update_state()"]

Also, fix a bug where the state of the channels is returned in data[0].
The comedi core expects it to be returned in data[1].

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>