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>