signal/ppc/{save,restore}_user_regs remove __put/get error checks

As __get_user and __put_user do not return errors, remove the
if checks from around them. This allows making the save/restore
functions void.

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Cc: Alexander Graf <agraf@suse.de>
