mtd: mtdoops: Fix kmsgdump parameter renaming.

When the kmsg_dumper callback parameter changed, the reason variable
in mtdoops_do_dump() was not updated accordingly.
This breaks the build with mtdoops.

Fixes: e1a261ba ("printk: Add a short description string to kmsg_dump()")
Suggested-by: Knop Ryszard <ryszard.knop@intel.com>
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Richard Weinberger <richard@nod.at>
Link: https://patchwork.freedesktop.org/patch/msgid/20240719152542.1554440-1-jfalempe@redhat.com