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>