net: hns3: replace snprintf with scnprintf in hns3_update_strings

snprintf returns the number of bytes that would be written, which may be
greater than the the actual length to be written. Here use extra code to
handle this.

scnprintf returns the number of bytes that was actually written, just use
scnprintf to simplify the code.

Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>