avr32: cast syscall_return to silence compiler warning

The patch fixes the following compiler warning:
    CC      arch/avr32/kernel/process.o
  arch/avr32/kernel/process.c: In function 'copy_thread':
  arch/avr32/kernel/process.c:292: warning: assignment makes integer \
  from pointer without a cast
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Hans-Christian Egtvedt <egtvedt@samfundet.no>

arch/avr32/kernel/process.c

@@ -289,7 +289,7 @@ int copy_thread(unsigned long clone_flags, unsigned long usp,
 		memset(childregs, 0, sizeof(struct pt_regs));
 		p->thread.cpu_context.r0 = arg;
 		p->thread.cpu_context.r1 = usp; /* fn */
-		p->thread.cpu_context.r2 = syscall_return;
+		p->thread.cpu_context.r2 = (unsigned long)syscall_return;
 		p->thread.cpu_context.pc = (unsigned long)ret_from_kernel_thread;
 		childregs->sr = MODE_SUPERVISOR;
 	} else {