net: hns3: clean up hns3_dbg_cmd_write()

As more commands are added, hns3_dbg_cmd_write() is going to
get more bloated, so move the part about command check into
a separate function.

Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>