dmi-id: Possible cleanup

The DEFINE_DMI_ATTR macro has a single user left so we can expand it
for slightly shorter/simpler code.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Lennart Poettering <mzxreary@0pointer.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>