tg3: Fix TSO test against wrong flags var

Julia Lawall discovered that the TG3_FLG2_TSO_CAPABLE flag was being
compared against the wrong flags device member.  This patch implements
the fix.

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>