iio: stx104: Allow build for X86_64

With the introduction of the ISA_BUS_API Kconfig option, ISA-style
drivers may be built for X86_64 architectures. This patch changes the
ISA Kconfig option dependency of the Apex Embedded Systems STX104 DAC
driver to ISA_BUS_API, thus allowing it to build for X86_64 as it is
expected to.

Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>