m68k: kernel: Use str_read_write() helper function

Remove hard-coded strings by using the str_read_write() helper function
and remove some unnecessary negations.

Compile-tested only.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/20241020205758.332095-1-thorsten.blum@linux.dev


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