PCI: pci-bridge-emul: Re-arrange register tests

Re-arrange the tests for which sets of registers are being accessed so that
it is easier to add further regions later. No functional change.

[pali: Fix reading old value in pci_bridge_emul_conf_write]
Link: https://lore.kernel.org/r/20220222155030.988-2-pali@kernel.org


Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>