net: hns3: add support to query tx spare buffer size for pf

Add support to query tx spare buffer size from configuration
file, and use this info to do spare buffer initialization when
the module parameter 'tx_spare_buf_size' is not specified.

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