input/atari: Use the correct mouse interrupt hook

The Atari keyboard driver calls atari_mouse_interrupt_hook if it's set, not
atari_input_mouse_interrupt_hook. Fix below.

[geert] Killed off atari_mouse_interrupt_hook completely, after fixing another
incorrect assignment in atarimouse.c.

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