net: tulip: de4x5: make PCI_signature() return void

This function always return 0 now, we can make it return void to
simplify the code. This fixes the following coccicheck warning:

drivers/net/ethernet/dec/tulip/de4x5.c:3908:11-17: Unneeded variable:
"status". Return "0" on line 3912

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>