virtio_pci_modern: Remove endianness conversion for config space

The bus_* functions already handle converting from PCI endianness
(i.e. little-endian) to native endianness when accessing the config
space (see ofw_pcib_bus_get_bus_tag), so converting again with
virtio_htogX/virtio_gtohX undoes any byte-swapping and breaks
big-endian systems. They should only be used for operating on shared
memory.
Note part of this reverts commit fb53b42e36a9 ("virtio-modern: fix PCI
common read/write functions on big endian targets").

PR: 294706
Reviewed by: adrian, tuexen
Fixes: fb53b42e36a9 ("virtio-modern: fix PCI common read/write functions on big endian targets")
Fixes: 9da9560c4dd3 ("virtio: Add VirtIO PCI modern (V1) support")
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D57392