bnxt_en: Do not enable legacy TX push on older firmware.

Older firmware may not support legacy TX push properly and may not
be disabling it.  So we check certain firmware versions that may
have this problem and disable legacy TX push unconditionally.

Fixes: c0c050c5

 ("bnxt_en: New Broadcom ethernet driver.")
Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>