asm-generic: drop HARDIRQ_BITS definition from hardirq.h

Architechtures normally don't need to set a HARDIRQ_BITS
unless they have hardcoded a specific value in assembly.
This drops the definition from asm-generic/hardirq.h, which
results in linux/hardirq.h setting its default of 10.

Both the old default of 8 and the linux/hardirq.h default
of 10 are sufficient because they only limit the number
of nested hardirqs, and we normally run out of stack space
much earlier than exceeding 256 or even 1024 nested interrupts.

Reported-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>