Fix unused variable warning

[nobodyinperson]

There is a compiler warning about the unused retval variable in void shell_task():

Compiling .pio/build/mega-station-debug/libbcc/GeekFactory Shell Library_ID1946/Shell.c.o                                                                                                                                                    
.pio/libdeps/mega-station-debug/GeekFactory Shell Library_ID1946/Shell.c: In function 'shell_task':                                                                                                                                          
.pio/libdeps/mega-station-debug/GeekFactory Shell Library_ID1946/Shell.c:290:6: warning: variable 'retval' set but not used [-Wunused-but-set-variable]                                                                                      
  int retval = 0;
      ^

This PR fixes this (and a couple of whitespace issues) by letting shell_task() return the retval which I assume you intended anyways.