PCI: endpoint: pci-epf-test: Limit PCIe BAR size for fixed BARs

Currently, the test allocates BAR sizes according to fixed table bar_size.
This does not work with controllers which have fixed size BARs that are
smaller than the requested BAR size. One such controller is Renesas R-Car
V4H PCIe controller, which has BAR4 size limited to 256 bytes, which is
much less than one of the BAR size, 131072 currently requested by this
test. A lot of controllers drivers in-tree have fixed size BARs, and they
do work perfectly fine, but it is only because their fixed size is larger
than the size requested by pci-epf-test.c

Adjust the test such that in case a fixed size BAR is detected, the fixed
BAR size is used, as that is the only possible option.

This helps with test failures reported as follows:

  pci_epf_test pci_epf_test.0: requested BAR size is larger than fixed size
  pci_epf_test pci_epf_test.0: Failed to allocate space for BAR4

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
[mani: reworded description]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Link: https://patch.msgid.link/20250905184240.144431-1-marek.vasut+renesas@mailbox.org