s390/setup: Fix secure ipl message

The new machine loader on z15 always creates an IPL Report block and
thus sets the IPL_PL_FLAG_IPLSR even when secure boot is disabled. This
causes the wrong message being printed at boot. Fix this by checking for
IPL_PL_FLAG_SIPL instead.

Fixes: 9641b8cc ("s390/ipl: read IPL report at early boot")
Signed-off-by: Philipp Rudo <prudo@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>