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: 247576b ("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>

Author: Alexander Gordeev

arch/s390/kernel/ipl.c

@@ -270,7 +270,7 @@ static ssize_t sys_##_prefix##_##_name##_store(struct kobject *kobj,	\
 	if (len >= sizeof(_value))					\
 		return -E2BIG;						\
 	len = strscpy(_value, buf, sizeof(_value));			\
-	if (len < 0)							\
+	if ((ssize_t)len < 0)						\
 		return len;						\
 	strim(_value);							\
 	return len;							\