[TG3]: Fix bug in setting a tg3_flag

Found a bug while reviewing the patches the second time.

The TG3_FLAG_TXD_MBOX_HWBUG flag is set after the register access
methods have been determined. This patch fixes it by moving it up before
the various access methods are assigned.

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