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 <mchan@broadcom.com>
Reported-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reported-by: Anisse Astier <anisse@astier.eu>
Suggested-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>