serial: sprd: Remove unnecessary resource validation

The devm_ioremap_resource() will valid the resources, thus remove the
unnecessary resource validation in the driver.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Acked-by: Chunyan Zhang <chunyan.zhang@spreadtrum.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>