ethtool: rss: move the device op invocation out of rss_prepare_data()

Factor calling device ops out of rss_prepare_data().
Next patch will add alternative path using xarray.
No functional changes.

Reviewed-by: Joe Damato <jdamato@fastly.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>