vmxnet3: allow rx flow hash ops only when rss is enabled

It makes sense to allow changes to get/set rx flow hash callback only
when rss is enabled. This patch restricts get_rss_hash_opts and
set_rss_hash_opts methods to allow querying and configuring different
Rx flow hash configurations only when rss is enabled

Signed-off-by: Ronak Doshi <doshir@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>