s390/ipl: Fix never less than zero warning

s390/ipl: Fix never less than zero warning

DEFINE_IPL_ATTR_STR_RW() macro produces "unsigned 'len' is never less
than zero." warning when sys_vmcmd_on_*_store() callbacks are defined.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202412081614.5uel8F6W-lkp@intel.com/
Fixes: 247576bf624a ("s390/ipl: Do not accept z/VM CP diag X'008' cmds longer than max length")
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>