net/fm10k: fix RSS hash config
net/fm10k: fix RSS hash config

Sometimes app just wants to update the RSS hash function and no RSS key
update is needed, but fm10k pmd will return EINVAL for this case.

If the rss_key is NULL, we don't need to check the rss_key_len.

Fixes: 57033cdf8fdc ("fm10k: add PF RSS")

Reported-by: Xueqin Lin <ee6f8db0c73be5c9a3b40f82a5086a34e3b5147a@intel.com>
Signed-off-by: Xiao Wang <28aecb4e45c1566b6185c24680691da84bd4fcf8@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk