iio: stx104: Utilize the module_isa_driver and max_num_isa_dev macros

The Apex Embedded Systems STX104 DAC drivers does not do anything
special in module init/exit. This patch eliminates the module init/exit
boilerplate code by utilizing the module_isa_driver macro.

Additionally, the max_num_isa_dev macro is utilized to simplify the
determination of maximum possible STX104 devices in the system.

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