[TG3]: more use of TG3_FLG2_5705_PLUS flag

Rewrite of a couple of troublesome multi-way if statements to use
TG3_FLG2_5705_PLUS flag.

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