[TG3]: use TG3_FLG2_5705_PLUS instead of multi-way if's

Replace a number of three-way if statements checking for 5705, 5750,
and 5752 to reference the equivalent TG3_FLG2_5705_PLUS flag instead.

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