s390/con3215: Remove spurious NULL in attribute_group definition

This NULL value is most-likely a copy-paste error from an array
definition. So far the NULL didn't have any effect.
As there will be a union in struct attribute_group at this location,
it will trigger a compiler warning.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20241118-sysfs-const-attribute_group-fixes-v1-2-48e0b0ad8cba@weissschuh.net


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>