mlxsw: spectrum_router: Pass ifindex to mlxsw_sp_ipip_entry_find_by_decap()

The sole caller of the function will soon only have the ifindex
available, instead of the pointer itself.

Therefore, change the function to take the ifindex as input and have it
get the pointer.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>