netdevsim: Create a helper for setting nexthop hardware flags

Instead of calling nexthop_set_hw_flags(), call a helper. It will be
used to also set nexthop bucket flags in a subsequent patch.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>