hwmon: (lm78) Make ISA interface depend on CONFIG_ISA

We should only include support for the ISA interface of the LM78/LM79
if CONFIG_ISA is set. Not only this makes the driver somewhat smaller
on most architectures, but this also avoids poking at random I/O
ports on these architectures.

This is very similiar to what was done for the w83781d driver in
October 2008.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Dean Nelson <dnelson@redhat.com>