floppy: add return value

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Makefile: add 'clean' target

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
floppy: clean up code

Remove unused variables, add return statements.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Makefile: include more warnings

Signed-off-by: Brian Norris <computersforpeace@gmail.com>