Atari keyboard: incorporate additional review comments

Atari keyboard: incorporate additional review comments:
  o Kill reference to source file name
  o Return error value from input_register_device() instead of -ENOMEM

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Michael Schmitz <schmitz@biophys.uni-duesseldorf.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>