ofproto: Fix ofport allocation.

Currently running the following 3 commands results
in vswitchd running in an infinite loop.

ovs-vsctl add-port br1 port1 -- set interface port1 \
type=internal ofport_request=1
ovs-vsctl add-port br1 port2 -- set interface port2 \
type=internal ofport_request=2
ovs-vsctl add-port br1 port3 -- set interface port3 \
type=internal

This patch lets us go through all the possible ofport
values to find a free ofport.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>