tg3: Add braces around 5906 workaround.

Commit dabc5c67, entitled
"tg3: Move TSO_CAPABLE assignment", moved some TSO flagging code around.
In the process it failed to add braces around an exceptional 5906
condition.  This patch fixes the problem.

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