net: hns3: optimize local variable initialization

The variable tx_ring is unnecessary to be initialized as it will be set
before used, and the variable rst_cnt is better to be initialized when
declaration for simplification.

Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>