tg3: Fix a flags typo

This patch fixes a problem where the TG3_FLAG_10_100_ONLY flag was
testing against the wrong flags variable.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>