s390/pkey: fix typos in comments

Various spelling mistakes in comments.
Detected with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Signed-off-by: Harald Freudenberger <freude@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>