bridge: Avoid reading other_config columns with ovsdb_idl_get().

bridge: Avoid reading other_config columns with ovsdb_idl_get().

ovsdb_idl_get() doesn't work with synthetic records.  Upcoming commits
will start synthesizing more ports and interfaces, so we should avoid
using ovsdb_idl_get().

In the long term it's probably a good idea to come up with a better way
to do synthetic database records, one that causes less trouble.

Bug #8457.