tg3: unconditionally select HWMON support when tg3 is enabled.

There is the seldom used corner case where HWMON=m at the same
time as TIGON3=y (typically randconfigs) which will cause a link
fail like:

drivers/built-in.o: In function `tg3_close':
tg3.c:(.text+0x16bd86): undefined reference to `hwmon_device_unregister'
drivers/built-in.o: In function `tg3_hwmon_open':
tg3.c:(.text+0x16fc4b): undefined reference to `hwmon_device_register'
make[1]: *** [vmlinux] Error 1

Fix it as suggested by DaveM[1] by having the Kconfig logic simply
select HWMON when TIGON3 is selected. This gets rid of all the
extra IS_ENABLED ifdeffery in tg3.c as a side benefit.

[1] http://marc.info/?l=linux-netdev&m=134250573718151&w=2

Cc: Michael Chan <[e-mail]>
Reported-by: Benjamin Herrenschmidt <[e-mail]>
Reported-by: Anisse Astier <[e-mail]>
Suggested-by: David S. Miller <[e-mail]>
Signed-off-by: Paul Gortmaker <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ethernet/broadcom/Kconfig

···
 	tristate "Broadcom Tigon3 support"
 	depends on PCI
 	select PHYLIB
+	select HWMON
 	---help---
 	  This driver supports Broadcom Tigon3 based gigabit Ethernet cards.
 

drivers/net/ethernet/broadcom/tg3.c

···
 #include <linux/prefetch.h>
 #include <linux/dma-mapping.h>
 #include <linux/firmware.h>
-#if IS_ENABLED(CONFIG_HWMON)
 #include <linux/hwmon.h>
 #include <linux/hwmon-sysfs.h>
-#endif
 
 #include <net/checksum.h>
 #include <net/ip.h>
···
 	return tg3_reset_hw(tp, reset_phy);
 }
 
-#if IS_ENABLED(CONFIG_HWMON)
 static void tg3_sd_scan_scratchpad(struct tg3 *tp, struct tg3_ocir *ocir)
 {
 	int i;
···
 	.attrs = tg3_attributes,
 };
 
-#endif
-
 static void tg3_hwmon_close(struct tg3 *tp)
 {
-#if IS_ENABLED(CONFIG_HWMON)
 	if (tp->hwmon_dev) {
 		hwmon_device_unregister(tp->hwmon_dev);
 		tp->hwmon_dev = NULL;
 		sysfs_remove_group(&tp->pdev->dev.kobj, &tg3_group);
 	}
-#endif
 }
 
 static void tg3_hwmon_open(struct tg3 *tp)
 {
-#if IS_ENABLED(CONFIG_HWMON)
 	int i, err;
 	u32 size = 0;
 	struct pci_dev *pdev = tp->pdev;
···
 		dev_err(&pdev->dev, "Cannot register hwmon device, aborting\n");
 		sysfs_remove_group(&pdev->dev.kobj, &tg3_group);
 	}
-#endif
 }