s390: fix comment regarding interrupts in svc

With the removal of the critical section cleanup, we now enter the svc
interrupt handler with interrupts disabled.

Fixes: 0b0ed657 ("s390: remove critical section cleanup from entry.S")
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>