tg3: Return size from tg3_vpd_readblock()

Newer VPD datablocks can exceed the size the tg3 driver is traditionally
used to.  This can cause some of the routines that operate on the VPD
data to fail when in-fact they could have succeeded had they known the
correct size.  This patch fixes the problem.

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