m68k: atari: Fix TT bootup freeze / unexpected (SCU) interrupt messages

Avoid freeze on Atari TT / MegaSTe boot with continuous messages of:

	unexpected interrupt from 112

Which was due to VBL interrupt being enabled in SCU sys mask, but there
being no handler for that any more.

(Bug and fix were first verified on real Atari TT HW by Christian,
 this patch later on in Hatari emulator.)

Fixes: 1fa0b29f ("fbdev: Kill Atari vblank cursor blinking")
Reported-by: Nicolas Pomarède <npomarede@corp.free.fr>
Closes: https://listengine.tuxfamily.org/lists.tuxfamily.org/hatari-devel/2024/06/msg00016.html
Closes: https://lore.kernel.org/all/9aa793d7-82ed-4fbd-bce5-60810d8a9119@helsinkinet.fi


Tested-by: Christian Zietz <czietz@gmx.net>
Signed-off-by: Eero Tamminen <oak@helsinkinet.fi>
Reviewed-by: Michael Schmitz <schmitzmic@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/20240624144901.5236-1-oak@helsinkinet.fi


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