target/s390x/tcg/mem_helper: Test the right bits in psw_key_valid()

target/s390x/tcg/mem_helper: Test the right bits in psw_key_valid()

The PSW key mask is a 16 bit field, and the psw_key variable is
in the range from 0 to 15, so it does not make sense to use
"0x80 >> psw_key" for testing the bits here. We should use 0x8000
instead.

Message-Id: <20221205142043.95185-1-thuth@redhat.com>
Reviewed-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>