revert "m68k: convert to asm-generic/hardirq.h"

Revert 45d80eea ("m68k: convert to
asm-generic/hardirq.h") - it fails to compile due to an inclusion tangle:

In file included from include/linux/irq.h:12,
                 from include/asm-generic/hardirq.h:6,
                 from /usr/src/devel/arch/m68k/include/asm/hardirq_mm.h:6,
                 from /usr/src/devel/arch/m68k/include/asm/hardirq.h:4,
                 from include/linux/hardirq.h:10,
                 from /usr/src/devel/arch/m68k/include/asm/system_mm.h:69,
                 from /usr/src/devel/arch/m68k/include/asm/system.h:4,
                 from include/linux/list.h:7,
                 from include/linux/preempt.h:11,
                 from include/linux/spinlock.h:50,
                 from include/linux/seqlock.h:29,
                 from include/linux/time.h:8,
                 from include/linux/timex.h:56,
                 from include/linux/sched.h:56,
                 from arch/m68k/kernel/asm-offs...