ofproto-dpif-upcall: Fix sparse warnings.

Fixes these warnings from "sparse":

../ofproto/ofproto-dpif-upcall.c:761:1: warning: symbol 'free_upcall' was
    not declared. Should it be static?
../ofproto/ofproto-dpif-upcall.c:849:1: warning: symbol 'convert_upcall'
    was not declared. Should it be static?

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>