be2net: make hwmon interface optional

The hwmon interface in the be2net driver causes a link error when
be2net is built-in while the hwmon subsystem is a loadable module:

drivers/built-in.o: In function `be_probe':
drivers/net/ethernet/emulex/benet/be_main.c:5761: undefined reference to `devm_hwmon_device_register_with_groups'

This adds a new Kconfig symbol, following the example of multiple
other drivers that have the same problem. The new CONFIG_BE2NET_HWMON
will not be available when (BE2NET=y && HWMON=m) to avoid this
problem.

We have to also mark be_hwmon_show_temp as 'static' to ensure the
compiler can optimize out all the unused code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 29e9122b3a ("be2net: Export board temperature using hwmon-sysfs interface.")
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ethernet/emulex/benet/be_main.c b/drivers/net/ethernet/emulex/benet/be_main.c
index dc7c0fd..75696d4 100644
--- a/drivers/net/ethernet/emulex/benet/be_main.c
+++ b/drivers/net/ethernet/emulex/benet/be_main.c
@@ -5612,9 +5612,9 @@
 	free_netdev(adapter->netdev);
 }
 
-ssize_t be_hwmon_show_temp(struct device *dev,
-			   struct device_attribute *dev_attr,
-			   char *buf)
+static ssize_t be_hwmon_show_temp(struct device *dev,
+				  struct device_attribute *dev_attr,
+				  char *buf)
 {
 	struct be_adapter *adapter = dev_get_drvdata(dev);
 
@@ -5756,7 +5756,7 @@
 	be_schedule_err_detection(adapter);
 
 	/* On Die temperature not supported for VF. */
-	if (be_physfn(adapter)) {
+	if (be_physfn(adapter) && IS_ENABLED(CONFIG_BE2NET_HWMON)) {
 		adapter->hwmon_info.hwmon_dev =
 			devm_hwmon_device_register_with_groups(&pdev->dev,
 							       DRV_NAME,