s390/ipl: use octal values instead of S_* macros

octal values are easier to read and checkpatch also recommends
to use them, so replace all the S_* macros with their counterparts.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>