mlxsw: spectrum_router: Fix arguments alignment

Suppress the following checkpatch.pl check [1] by adding a variable to
store the IP-in-IP options. Noticed while adding equivalent IPv6 code in
subsequent patches.

[1]
CHECK: Alignment should match open parenthesis
+               mlxsw_reg_ritr_loopback_ipip4_pack(ritr_pl, lb_cf.lb_ipipt,
+
+ MLXSW_REG_RITR_LOOPBACK_IPIP_OPTIONS_GRE_KEY_PRESET,

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