[TG3]: use TG3_FLG2_57{05,50}_PLUS flags in tg3_get_invariants

Rewrite checks in tg3_get_invariants to use TG3_FLG2_5705_PLUS and
TG3_FLG2_5750_PLUS flags.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>