tg3: Reclaim TG3_FLG3_5761_5784_AX_FIXES flag

This patch reclaims the TG3_FLG3_5761_5784_AX_FIXES flag.  It only
used twice in non-fast paths.  This patch also consolidates some other
places where specific 5784 AX chip revisions can be generalized.

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