ofproto: Merge all the CFM query functions into one.

This eliminates several function calls and in particular several
hash table lookups to find structures corresponding to a port
number from iface_refresh_cfm_stats().

This does not seem to reduce CPU use, but the code is shorter
and more readable.

Bug #15171.
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>