Simplify ofproto_controller_info by using a struct smap in place of array.

The only client for ofproto_controller_info was transforming the array of
pairs into an smap anyway.  It's easy for the code that fills in the array
to generate it as an smap directly, and it's also easier to extend later.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Gurucharan Shetty <gshetty@nicira.com>