tg3: Add the ability to conditionally build w/ HWMON

Introduce a Kconfig option: CONFIG_TIGON3_HWMON which allows to build
in/out support for thermal sensors reported by Tigon3 NICs.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>