comedi vmk80xx: make rudimentary_check a static function

rudimentary_check is a macro with side-effects (it returns
on error) which is contary to CodingStyle. Replace it with
a static function.

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