mtd: core: tone down suggestion that dev.parent should be set

add_mtd_device() has a comment suggesting that the caller should have
set dev.parent. This is required to have the parent device symlink show
up in sysfs, but not for proper operation of the mtd device itself.
Currently we have five drivers registering mtd devices during module
initialization, so they don't actually provide a parent device to link
to. That means we cannot WARN_ON() here, as it would trigger false
positives.

Make the comment a bit less firm in its assertion that dev.parent should
be set.

Signed-off-by: Frans Klaver <fransklaver@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>