Fix logic for reading parameter in MOM_tracer_advect.F90

We had random behavior in the doc files because the logical `CS%useHuynh`
was not set when using the PLM scheme.

src/tracer/MOM_tracer_advect.F90

@@ -1119,6 +1119,8 @@ subroutine tracer_advect_init(Time, G, US, param_file, diag, CS)
           "  PPM:H3 - Piecewise Parabolic Method (Huyhn 3rd order)\n"// &
           "  PPM    - Piecewise Parabolic Method (Colella-Woodward)" &
           , default='PLM')
+  CS%usePPM = .false.
+  CS%useHuynh = .false.
   select case (trim(mesg))
     case ("PLM")
       CS%usePPM = .false.
@@ -1127,13 +1129,12 @@ subroutine tracer_advect_init(Time, G, US, param_file, diag, CS)
       CS%useHuynh = .true.
     case ("PPM")
       CS%usePPM = .true.
-      CS%useHuynh = .false.
     case default
       call MOM_error(FATAL, "MOM_tracer_advect, tracer_advect_init: "//&
            "Unknown TRACER_ADVECTION_SCHEME = "//trim(mesg))
   end select
 
-  if (CS%useHuynh) then
+  if (CS%usePPM .and. CS%useHuynh) then
     call get_param(param_file, mdl, "USE_HUYNH_STENCIL_BUG", &
         CS%useHuynhStencilBug, &
         desc="If true, use a stencil width of 2 in PPM:H3 tracer advection. " &