pci: renesas: Fix BAR mapping on Gen3

Because the first PCIExAR(n) register is configured with the mapping,
It is the second PCIExAR(n) register that must be written with 0, not
the last one. Update the n from 4 to 1 to select the correct register.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>