mlxsw: spectrum_router: Add a helper to check if netdev has addresses

This function will be useful later as the driver will need to retroactively
create RIFs for new uppers with addresses.

Add another helper that assumes RCU lock, and restructure the code to
skip the IPv6 branch not through conditioning on the addr_list_empty
variable, but by directly returning the result value. This makes the skip
more obvious than it previously was.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>