fbcon: Protect free_irq() by MACH_IS_ATARI check

Add missing check for Atari in free_irq() call, which could cause problems on
multi-platform m68k kernels.

Reported-by: Brad Boyer <flar@allandria.com>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>