drivers/net/tg3.c: change the field used with the TG3_FLAG_10_100_ONLY constant

The constant TG3_FLAG_10_100_ONLY should be used with the tg3_flags field,
not the tg3_flags2 field, as done elsewhere in the same file.

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>