meta-flow: Make it simpler to expand mf_values in the future

Remove hard coded array index and make it dependent on the array size.

Signed-off-by: Madhu Challa <challa@noironetworks.com>
Co-authored-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>