fbdev: c2p - Cleanups

  - Improve comments and naming
  - Convert macros to static inline functions
  - Remove superfluous `break' after `return'
  - Make sure we get a build-time error (undefined reference to
    'c2p_unsupported') in case of future misuse
  - Replace `unsigned long' by `u32' in comp(), as that's what all callers use
  - Use {get,put}_unaligned_be32() in store_planar{,_masked}()
  - Use void * for arbitrary pointers
  - Use a union to represent pixels/words, to avoid casts

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>