tg3: fix return value check in tg3_read_vpd()

Besides -ETIMEDOUT and -EINTR, pci_read_vpd may return other error
values like -ENODEV or -EINVAL which are ignored due to the buggy
check, but the data are not read from VPD anyway and this is checked
subsequently with at most 3 needless loop iterations. This does not
show up as a runtime bug.

CC: Matt Carlson <mcarlson@broadcom.com>
CC: Michael Chan <mchan@broadcom.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>