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>

include/linux/hw_breakpoint.h

@@ -93,7 +93,7 @@ register_user_hw_breakpoint(struct perf_event_attr *attr,
 			    struct task_struct *tsk)	{ return NULL; }
 static inline int
 modify_user_hw_breakpoint(struct perf_event *bp,
-			  struct perf_event_attr *attr)	{ return NULL; }
+			  struct perf_event_attr *attr)	{ return 0; }
 static inline struct perf_event *
 register_wide_hw_breakpoint_cpu(struct perf_event_attr *attr,
 				perf_overflow_handler_t	 triggered,