ofp-util: Reduce duplicate code.

ofp-util: Reduce duplicate code.

ofputil_put_phy_port() and ofputil_append_port_desc_stats_reply() had a
lot of code duplication.  This reduces it: it deletes some specialized
code from ofputil_put_phy_port(), moving it into its caller
ofputil_put_switch_features_port() that actually needed it.  That change
then allows ofputil_append_port_desc_stats_reply() to become a simple
wrapper around ofputil_put_phy_port().

Signed-off-by: Ben Pfaff <blp@nicira.com>