misc: pci_endpoint_test: Avoid using hard-coded BAR sizes

BAR sizes are hard-coded in pci_endpoint_test driver corresponding to the
sizes used in pci-epf-test function driver. This might break if the sizes
in pci-epf-test function driver are modified (and the corresponding change
is not done in pci_endpoint_test PCI driver).

To avoid hard coding BAR sizes, use pci_resource_len() API.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>