datapath-windows: update CodingStyle guideline for variable names

During a review, it seemed that some of the conventions were not clear.
Fixing them in this patch.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Reported-by: Samuel Ghinet <sghinet@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Samuel Ghinet <sghinet@cloudbasesolutions.com>