In ELM `Flrr_flood` should only be accounted for when two-way land-river coupling is turned on

Currently, the flux of flood water from MOSART (i.e. `Flrr_flood`) is being added even when the two-way land-river coupling is not turned on. Technically, the `Flrr_flood` is supposed to be `0.0_r8` for the default run (i.e. one-way land-river coupling). However, @tanzeli1982 found out that when MOSART was blowing up due to numerical instability because the MOSART time step was too large, `Flrr_flood` was not `0.0_r8`. The fix is to only add `Flrr_flood` in ELM when the two-way land-river coupling is turned on.

cc: @tanzeli1982 , @liho745, @hydrotian