staging: iio: adc: Replace of_iomap() with devm_ioremap_resource()

The adc driver uses of_iomap() which doesn't request the resource and
isn't device managed so error handling is needed. of_iomap() is mainly
used in cases where there is no driver or struct device so a switch to
devm_ functions is required.

This patch switches to use devm_ioremap_resource() instead which
automatically requests the resource and is freed when the driver
detaches.

Removed the error handling to unmap I/O registers i.e.
iounmap() in probe and remove functions of this driver and
consequently removed an unnecessary label.

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>