tg3: sparse cleanup

Fix the following sparse warning :

drivers/net/tg3.c:4025:3: warning: context imbalance in 'tg3_restart_hw' 
- unexpected unlock

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>