virtio: fix get_config / set_config for legacy VirtIO targets

The functions virtio_pci_get_config() and virtio_pci_set_config() don't
take the offset into account when reading the config space. For example
this manifests when U-Boot tries to read the MAC address of the VirtIO
networking device. It reads 6 equa bytes instead of the proper addess.

Fix those functions by taking the offset in the config space into
account.

Fixes: 4135e107 ("virtio: Add virtio over pci transport driver")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>