xtensa: fix prefetch in the raw_copy_to_user

'from' is the input buffer, it should be prefetched with prefetch, not
prefetchw.

Tested-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>