[TG3]: Add missing unlock in tg3_open() error path.

Sparse noticed a locking imbalance in tg3_open(). This patch adds an
unlock to one of the error paths, so that tg3_open() always exits
without the lock held.

Signed-off-by: Ira W. Snyder <kernel@irasnyder.com>
Signed-off-by: David S. Miller <davem@davemloft.net>