input/atari: Fix atarimouse init

Atarimouse fails to load as a module (with ENODEV), due to a brown paper
bag bug, misinterpreting the semantics of atari_keyb_init().

[geert] Propagate the return value of atari_keyb_init() everywhere

Signed-off-by: Michael Schmitz <schmitz@debian.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>