misc: pci_endpoint_test: Fix test_reg_bar to be updated in pci_endpoint_test

commit 834b9051 ("misc: pci_endpoint_test: Add support for
PCI_ENDPOINT_TEST regs to be mapped to any BAR") while adding
test_reg_bar in order to map PCI_ENDPOINT_TEST regs to be mapped to any
BAR failed to update test_reg_bar in pci_endpoint_test, resulting in
test_reg_bar having invalid value when used outside probe.

Fix it.

Fixes: 834b9051 ("misc: pci_endpoint_test: Add support for PCI_ENDPOINT_TEST regs to be mapped to any BAR")
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>