net: hns3: reallocate tx/rx buffer after changing mtu

When changing the mtu, the max frame size also will be changed. The tx
buffer size and the rx buffer size to be allocated are determined by max
frame size. So when max frame size is changed, the tx buffer and rx buffer
need to be reallocated.

When the tc_num is changed, the tx buffer and rx buffer need to be
reallocated too. So calling set_mtu and buffer_alloc separately is better.

Signed-off-by: Fuyun Liang <liangfuyun1@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>