CAM5 UW PBL: clean up eddy_diff.F90 to match coding standards

During review of #370, @nusbaume originally wrote:
> This Fortran module contains code that is generally not in agreement with our Fortran coding standards and style guide (e.g. the un-needed return here). Instead of reviewing it all now, maybe it would be worth using an AI once our "agents" file has been added to see if it can clean it all up? Again just noting it here in case we want to add it to the "future work" issue. Thanks!