[S390] cmm: fix crash on case conversion

When the cmm module is compiled into the kernel it will crash when
writing to the R/O data section.
Reason is the lower to upper case conversion of the "sender" module
parameter which ignored the fact that the pointer is preinitialized.

Introduced with 41b42876 "cmm, smsgiucv_app: convert sender to
uppercase"

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>