prescribed n uptake rate used for prescribed p (bug)

[rgknox]

@jingtao-lbl identified that prescribed p uptake is actually using the prescribed n parameter, see here:

https://github.com/NGEET/fates/blob/sci.1.92.4_api.45.0.0/biogeochem/FatesSoilBGCFluxMod.F90#L221

The fix has been added to #1558