ipq40xx: Fix Linksys upgrade, restore config step

ipq40xx: Fix Linksys upgrade, restore config step

It appears that the refactor of the upgrade process for NAND devices
resulted in the nand_do_upgrade_success step not being called for
devices using the linksys.sh script. As a result, configuration
was not preserved over sysupgrade steps.

This was restored for some devices in
commit 84ff6c90dda1 ("base-files: bring back nand_do_upgrade_success").
This restored preservation of config for ipq40xx devices using the
linksys.sh script. Other devices and targets have not been examined.

Closes: #11677
Fixes: e25e6d8e54 ("base-files: fix and clean up nand sysupgrade code")
Tested-on: EA8300

Signed-off-by: Jeff Kletsky <git-commits@allycomm.com>
(checkpatch nitpick)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>