tg3: Obtain PCI function number from device

This patch adds code to attempt to obtain the PCI function number from
the device rather than accept the number handed by the kernel.  In
pass-through scenarios, the function number handed by the kernel may not
reflect the true function of the device.

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