spi: ath79: simplify iomem resource mapping

Simplify the code by switching from devm_ioremap to devm_ioremap_resource.
Checking the result of platform_get_resource is not needed as
devm_ioremap_resource checks the provided resource argument for NULL.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>