tg3: Improve PCI function number detection.

Simplify the code to detect PCI function number on 5717, 5719, and 5720.
If shared memory does not have proper signature, read the function number
from register directly.

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