pci: sandbox: swap_case: Preserve space indicator bit in BAR registers

With the newly added testing of more than one device, we get:

  => ut dm pci_swapcase
  Test: dm_test_pci_swapcase: pci.c
  test/dm/pci.c:88, dm_test_pci_swapcase(): "tHIS IS A tESt" = ptr:
  Expected "tHIS IS A tESt", got "this is a test"
  Test: dm_test_pci_swapcase: pci.c (flat tree)
  test/dm/pci.c:88, dm_test_pci_swapcase(): "tHIS IS A tESt" = ptr:
  Expected "tHIS IS A tESt", got "this is a test"
  Failures: 2

The failure only happens on the 2nd swap_case device on the PCI bus.
The case passes on the 1st device.

It turns out the swap_case driver does not emulate bit#0 in BAR
registers as a read-only bit. This corrects the implementation.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>