FRV: Fix no-hardware-breakpoint case

If there is no hardware breakpoint support, modify_user_hw_breakpoint()
tries to return a NULL pointer through as an 'int' return value:

  In file included from kernel/exit.c:53:
  include/linux/hw_breakpoint.h: In function 'modify_user_hw_breakpoint':
  include/linux/hw_breakpoint.h:96: warning: return makes integer from pointer without a cast

Return 0 instead.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>