serial: sa1100: use platform_get_resource()

Use platform_get_resource() to fetch the memory resource
instead of open-coded variant.

While here, fail the probe if no resource found or no port is added.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200804134807.11589-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>