Fix host state checking logic

Resolves openbmc/bmcweb#12

Testing: Verified this with system booted against
redfish/v1/Systems/motherboard:
  "PowerState": "On",
  "Status": {
    "State": "Enabled"
  },
and this with system powered off:
  "PowerState": "Off",
  "Status": {
    "State": "Disabled"
  },

Change-Id: I0acbf0992d730e4a5189d9af962370277f59eaaa
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>