s390/cmm: use string_upper() instead of open coded variant

Use string_upper() from string helper module instead of open coded variant.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20211001130201.72545-1-andriy.shevchenko@linux.intel.com


[hca@linux.ibm.com: removed hunk which converts extmem.c]
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>