ixgbe: check whether thermal sensor is enabled.

The X540's internal thermal sensor should not be enabled for all devices, but
only those devices which enable it in the NVM image. It is expected that
actively cooled devices will have it enabled, but passively cooled devices might
not want it enabled. This is due to passively cooled devices operating very near
the thermal threshold, sometimes within the margin of error of the thermal
sensor. Thus these devices may not be good candidates for using the thermal
sensor.

This patch uses the enabled bit in the FWSM register to check whether we should
be enabling the thermal sensor, and only sets the THERMAL_SENSOR_CAPABLE flag
for those devices which have it enabled.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>