dpif-linux.c: Let the kernel pick a port number if one not requested.

dpif-linux.c: Let the kernel pick a port number if one not requested.

With the single datapath change, we no longer depend on the kernel to
make sure that we don't reuse OpenFlow port numbers, since the ofproto
library now picks them.  Remove the code that contained that logic.

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