[TG3]: misc. fixes

Fix interrupt test handler by adding check for IRQ assertion in
PCI_STATE register in addition to the status block updated bit.

Add test for valid ethernet address in tg3_set_mac_addr().

Add tg3_bus_string() to setup the PCI bus speed/width string for all
PCI/PCIX/PCI Express devices. This is used to print the bus type
during init_one().

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>