virtio: pci: fix bug of virtio_pci_map_capability

The bar of the structure virtio_pci_cap is the index, and each base
address occupies 4 bytes, so it needs to be multiplied by 4.

This patch fixes a bug reported by Felix Yan
https://lists.denx.de/pipermail/u-boot/2022-August/492779.html



Signed-off-by: Xiang W <wxjstz@126.com>
Tested-by: Felix Yan <felixonmars@archlinux.org>