Blackfin: define HARDIRQ_BITS again for now

The default values of HARDIRQ_BITS and PREEMPT_BITS in common code leads to
build failure:

In file included from include/linux/interrupt.h:12,
                from include/linux/kernel_stat.h:8,
                from arch/blackfin/kernel/asm-offsets.c:32:
include/linux/hardirq.h:66:2: error: #error PREEMPT_ACTIVE is too low!

So until that gets resolved, just declare our own default value again.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>