iio: stx104: Unregister IIO device on remove callback

The devm_iio_device_register function should not be used if custom
operations must be performed in the remove callback. This patch replaces
the dem_iio_device_register call with a iio_device_register call and
respective iio_device_unregister call in the remove callback.

Fixes: 765550e4 ("iio: stx104: Add GPIO support for the Apex Embedded Systems STX104")
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>