ethernet: Remove redundant statement

The variable will be assigned again later in the if condition,
there is no meaning there.

drivers/net/ethernet/broadcom/tg3.c:5750:2 warning:

Value stored to 'current_link_up' is never read.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: luo penghao <luo.penghao@zte.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>