xtensa: fix unaligned usermode access

- correct use of .config #define name;
      CONFIG_UNALIGNED_USER ---> CONFIG_XTENSA_UNALIGNED_USER

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Pete Delaney <piet@tensilica.com>
Signed-off-by: Chris Zankel <chris@zankel.net>