ethtool: Validate master slave configuration before rtnl_lock()

Create a new function for input validations to be called before
rtnl_lock() and move the master slave validation to that function.

This would be a cleanup for next patch that would add another validation
to the new function.

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>