s390x: Fix integer literal in skey.c

The code is a 64bit number of which the upper 48 bits must be 0.

Fixes: 965e38a0 ("s390x: Test effect of storage keys on diag 308")
Signed-off-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Message-Id: <20230104175950.731988-1-nsg@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>