microblaze: Use unsigned return type in do_syscall_trace_enter

Registers are not signed types.

The patch removes warnings:
arch/microblaze/kernel/ptrace.c: In function 'do_syscall_trace_enter':
arch/microblaze/kernel/ptrace.c:152:14: warning: signed and unsigned
type in conditional expression [-Wsign-compare]
  return ret ?: regs->r12;
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/include/asm/syscall.h

@@ -97,7 +97,7 @@ static inline void syscall_set_arguments(struct task_struct *task,
 		microblaze_set_syscall_arg(regs, i++, *args++);
 }
 
-asmlinkage long do_syscall_trace_enter(struct pt_regs *regs);
+asmlinkage unsigned long do_syscall_trace_enter(struct pt_regs *regs);
 asmlinkage void do_syscall_trace_leave(struct pt_regs *regs);
 
 static inline int syscall_get_arch(void)

arch/microblaze/kernel/ptrace.c

@@ -132,9 +132,9 @@ long arch_ptrace(struct task_struct *child, long request,
 	return rval;
 }
 
-asmlinkage long do_syscall_trace_enter(struct pt_regs *regs)
+asmlinkage unsigned long do_syscall_trace_enter(struct pt_regs *regs)
 {
-	long ret = 0;
+	unsigned long ret = 0;
 
 	secure_computing_strict(regs->r12);