ixgbe: cleanup the hwmon function calls

When the hwmon code was initially added it was with the assumption that a
sysfs patch would be also coming soon.  Since that isn't the case some
clean up needs to be done.  This patch does that.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>