tg3: Remove redundant assignments

The assignment of err will be overwritten next, so this statement
should be deleted.

The clang_analyzer complains as follows:

drivers/net/ethernet/broadcom/tg3.c:5506:2: warning:

Value stored to 'expected_sg_dig_ctrl' 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>