Move value interface creation to a helper method.
Move value interface creation to a helper method.

Change-Id: Id383f18e681a30f41976c07be1c7c9929af6abb5
Signed-off-by: Brad Bishop <713d098c0be4c8fd2bf36a94cd08699466677ecd@fuzziesquirrel.com>

openbmc/phosphor-hwmon,openbmc/phosphor-hwmon