Fix misplaced stopf in CNDriverMod.F90

[olyson]

Description of changes

In CNDriverMod.F90, there is a t_startf call that is within an if statement, but the t_stopf is outside the if statement. Reported by @olyson , solution by @mvdebolskiy

Specific notes

Contributors other than yourself, if any: @mvdebolskiy

CTSM Issues Fixed (include github issue #): #2291

Are answers expected to change (and if so in what way)? No

Any User Interface Changes (namelist or namelist defaults changes)? No

Testing performed, if any: Verified elimination of PET files in run directory.