comedi vmk80xx: extend rudimentary_check to check both directions

rudimentary_check() can currently check whether the input or
output direction is currently available (no pending transaction),
but not both at the same time. We need this facility for do_bits().

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