#58 Fix files.h return values enum

[Kimmo Varis]

In files.h there is a en enum for files function return values. The enum defines several "main" return values and one "modifier". The idea has been that return value is a combination of main value + modifier.

However, there are several "main" values that really should be main + modifier combinations, like FRESULT_OK_IMPURE should be OK main + IMPURE modifier.