ofproto-dpif: Disassociate datapath max_ports with openflow port numbers.

With single datapath, multiple userspace bridges share the same datapath.
As such it does not look beneficial that we decide a valid open flow port
number based on the number of ports in the datapath specially now that
we have the ofport_request column in OVSDB.

This commit does not remove ofproto_init_max_ports() interface as defined
in ofproto-provider.h as there may be other implementations that still use it.
But ofproto-dpif should not need it.

Bug #20163.
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>