s390/uaccess: add default cases for __put_user_fn()/__get_user_fn()

Add default cases for __put_user_fn()/__get_user_fn(). This doesn't
fix anything since the functions are only called with sane values.

However we get rid of smatch warnings:
./arch/s390/include/asm/uaccess.h:143 __get_user_fn() error: uninitialized symbol 'rc'.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>