Remove unnecessary check (#4528)

Author: gpotter2

scapy/arch/linux/__init__.py

@@ -142,8 +142,6 @@ def attach_filter(sock, bpf_filter, iface):
 def set_promisc(s, iff, val=1):
     # type: (socket.socket, _GlobInterfaceType, int) -> None
     _iff = resolve_iface(iff)
-    if not _iff.is_valid():
-        raise OSError("set_promisc: Unknown interface %s" % iff)
     mreq = struct.pack("IHH8s", _iff.index, PACKET_MR_PROMISC, 0, b"")
     if val:
         cmd = PACKET_ADD_MEMBERSHIP