pci: Adjust dm_pci_read_bar32() to return errors correctly

At present if reading a BAR returns 0xffffffff then the value is masked
and a different value is returned. This makes it harder to detect the
problem when debugging.

Update the function to avoid masking in this case.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>