net: hns3: Fixes the static check warning due to missing unsupp L3 proto check

This patch fixes the static check warning due to missing handling leg of
unsupported L3 protocol type in the hns3_get_l4_protocol() function.

Fixes: 76ad4f0e

 ("net: hns3: Add support of HNS3 Ethernet Driver for
hip08 SoC")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>