[TG3]: use new TG3_FLG2_5750_PLUS flag

Replace a number of two-way if statements checking for 5750, and/or
5752 to reference the newly-defined TG3_FLG2_5750_PLUS flag instead.

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