net/sfc: make flow RSS details VNIC-specific

The flow specification structure will be transformed to a generic one, and
its current contents will be fenced off to form a VNIC-specific parameters.
Flow RSS details do not belong to the said specification currently, albeit
being VNIC-specific. This patch addresses this issue as a preparation step.

Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>