hwmon: (w83781d) Make ISA interface depend on CONFIG_ISA

Probing the ISA bus on systems without ISA bus may hang the system.
This patch makes the ISA bus related code depend on the kernel
configuration parameter CONFIG_ISA. It moves ISA bus related code
into one #ifdef CONFIG_ISA ... #endif block and adds some helper
function.
Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>