comedi vmk80xx: support bits instruction

Calling comedi_dio_bifield2() returns EBUSY permanently. Implementing
the insn_bits call fixes the problem and is good in its own right since
one can then read and write to all the digitial lines at the same time.
Tested on a K8055, but not on a K8061.

Signed-off-by: J. Ali Harlow <ali@avrc.city.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>