vfio/pci: Drop unnecessary pcibios_err_to_errno()

As of commit d97ffe23

 ("PCI: Fix return value from
pci_user_{read,write}_config_*()") it's unnecessary to call
pcibios_err_to_errno() to fixup the return value from these functions.

pcibios_err_to_errno() already does simple passthrough of -errno values,
therefore no functional change is expected.

[aw: changelog]
Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>