microblaze: Fix sparse warnings - signal.c

Warning log:
CHECK   arch/microblaze/kernel/signal.c
arch/microblaze/kernel/signal.c:200:9: warning: Using plain integer as NULL pointer
arch/microblaze/kernel/signal.c:201:9: warning: incorrect type in initializer (different address spaces)
arch/microblaze/kernel/signal.c:201:9:    expected void [noderef] *volatile __gu_val<asn:1>
arch/microblaze/kernel/signal.c:201:9:    got void *<noident>

Signed-off-by: Michal Simek <monstr@monstr.eu>

arch/microblaze/kernel/signal.c

@@ -197,8 +197,8 @@ static void setup_rt_frame(int sig, struct k_sigaction *ka, siginfo_t *info,
 
 	/* Create the ucontext. */
 	err |= __put_user(0, &frame->uc.uc_flags);
-	err |= __put_user(0, &frame->uc.uc_link);
-	err |= __put_user((void *)current->sas_ss_sp,
+	err |= __put_user(NULL, &frame->uc.uc_link);
+	err |= __put_user((void __user *)current->sas_ss_sp,
 			&frame->uc.uc_stack.ss_sp);
 	err |= __put_user(sas_ss_flags(regs->r1),
 			&frame->uc.uc_stack.ss_flags);