s390/cpum_sf: Fix format string in pr_err()

Fix format string in pr_err() and use the built-in
hexadecimal prefix %#x to display a number with a leading
hexadecimal indicator 0x.
No functional change.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Reviewed-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>