iio: Change ISA_BUS_API dependency to selection

The ISA_BUS_API Kconfig option enables the compilation of the ISA bus
driver. The ISA bus driver does not perform any hardware interaction,
and is instead just a thin layer of software abstraction to eliminate
boilerplate code common to ISA-style device drivers. Since ISA_BUS_API
has no dependencies and does not jeopardize the integrity of the system
when enabled, drivers should select it when the ISA bus driver
functionality is needed.

Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>