KVM: s390: fix 24 bit psw handling in lpsw/lpswe handler

When checking for validity the lpsw/lpswe handler check that only
the lower 20 bits instead of 24 bits have a non-zero value.
There handling valid psws as invalid ones.
Fix the 24 bit psw mask.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>