Add StopWhenUnneeded to hwmon sd unit

The net effect is that systemd will stop the unit when a udev
remove event occurs.

Change-Id: I89413770b01c236caef74ca8af21d3b05845db45
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/common/recipes-phosphor/sensors/phosphor-hwmon/xyz.openbmc_project.Hwmon@.service b/common/recipes-phosphor/sensors/phosphor-hwmon/xyz.openbmc_project.Hwmon@.service
index fcfada6..a59349d 100644
--- a/common/recipes-phosphor/sensors/phosphor-hwmon/xyz.openbmc_project.Hwmon@.service
+++ b/common/recipes-phosphor/sensors/phosphor-hwmon/xyz.openbmc_project.Hwmon@.service
@@ -1,6 +1,7 @@
 [Unit]
 Description=Phosphor Hwmon Poller
 ConditionFileNotEmpty=/etc/default/obmc/hwmon/%I.conf
+StopWhenUnneeded=true
 
 [Service]
 Restart=always