tg3: Modify tg3_tso_bug() to handle multiple TX rings

tg3_tso_bug() was originally designed to handle only HW TX ring 0, Commit
d3f6f3a1d818410c17445bce4f4caab52eb102f1 ("tg3: Prevent page allocation failure
during TSO workaround") changed the driver logic to use tg3_tso_bug() for all
HW TX rings that are enabled. This patch fixes the regression by modifying
tg3_tso_bug() to handle multiple HW TX rings.

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