KVM: Fix PCI header check on device assignment

The masking was wrong (must have been 0x7f), and there is no need to
re-read the value as pci_setup_device already does this for us.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=43339


Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>